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Human Anatomy and Physiology Premium Course

Human Anatomy and Physiology premium is credited to James Ross, which help to solve some of the anatomy related issues which pose a significant challenge to the student and also young doctors. The product contains more 3000 pages with illustration and even graphics, which enhances a better understanding of the various topics in human physiology and also anatomy. It comprises of the component systems which covers many lessons. This product has been used by different practitioners and also a scholar in enhancing some better understanding of the topics in anatomy. Some of the illustrations and even graphics which are covered in the product have been researched and contain more detailed explanations which would rely upon. The product is available in the E-book, and one can download and also enjoy some bonus which would enhance some better understanding of the various topic which keeps on giving some of the problems too many people. More here...

Human Anatomy and Physiology Premium Course Summary


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Pilumcephalus Velos Lancehead

Use the rest of the skull as a club, either a regular one or as a flail, the skull being attached to the animal's flexible spine. Because this creature's aggressive behavior makes it an ideal guard animal for enclosed compounds where it doesn't have to obey a trainer's commands, its eggs are worth 100.

The evolution of Thyreophora

Ankylosaur Temporal Fenestra

Stegosauria is a monophyletic clade of ornithischian dinosaurs, diagnosed on the basis of a number of important features shown in Figure 5.22. The ancestral stegosaur must have been an animal with spine-shaped osteoderms and fore- and hindlimbs of not too dissimilar lengths. Within Stegosauria, the basal split is between Huayangosaurus on the one hand and remaining species on the other (Figure 5.23). This divergence took place sometime before the latter half of the Middle Jurassic. Huayangosaurus itself has a number of uniquely derived features shared by a more inclusive group of stegosaurs, Stegosauridae (Figure 5.23) Figure 5.22. Cladogram emphasizing the monophyly of Eurypoda and Stegosauria. Derived characters include at 1 (Eurypoda), bones that fuse to the margins of the eye sockets, loss of a notch between the quadrate (see Figure 4.6) and the back of the skull, and enlargement of the anterior part of the ilium at 2 (Stegosauria), back vertebrae with very tall neural arches and...

Pinacosaurus Skeletal

Forest Armadillo Lizard

As their name implies, ankylosaurs (ankylo - fused saurus - lizard) were encased in shell-like dermal armor (Figure 7.1), a pavement of bony plates and spines - each embedded in skin and interlocked with adjacent plates - that formed a continuous shield across the neck, throat, back, and tail. In many cases, it covered the top of the head and cheeks. The armament varied from species to species, and is used to identify particular ankylosaurs, even from scrappy material. Members of Nodosauridae had relatively long snouts, well-muscled shoulders, flaring hips, and pillar-like limbs. All of course were armor covered and many also had tall spikes and spines on their backs and shoulders. Nodosaurids are known principally from the Northern Hemisphere (North America and Europe), although new discoveries in Australia and Antarctica have extended the geographical range of these animals deep into the Southern Hemisphere. Members of Ankylosauridae also give the impression of impregnability. All...

The Pachycephalosauria Boneheaded Dinosaurs

The bipedal posture of pachycephalosaurs was aided by a spine that locked the vertebrae together for strength and flexibility. The animals walked in a leaning position, with their back parallel to the ground. Because the upper part of the pachycephalosaur leg (the femur) was longer than the lower part (the tibia and fibula), these dinosaurs were probably not fast runners.

Eurypoda Stegosauria hot plates

Stegosaurus Tooth

Stegosaurs were medium-sized dinosaurs, 3-9 m in length and weighing 300-1,500 kg, characterized by osteoderms that developed into spines and plates, as well as by their quadrupedal stance (Figure 5.3). Their profiles sloped strongly forward and downward toward the ground as a result of the hindlimbs being substantially longer than the forelimbs (Figure 5.4). All toes had broad hooves. They seem to have been relatively uncommon dinosaurs, yet clearly had a global distribution (Figure 5.5). No brains, one brain, or two brains It is as clear as most anything can be at a distance of 100 million years that stegosaurs were just not all that bright. Their brains were an estimated 0.001 of the adult stegosaur body weight, putting them near the bottom of the dinosaur -for that matter, vertebrate - gray-matter scale (Figure 5.9). Brainy-ness must not have been part of the stegosaur life strategy, as indeed they were so small-brained that early workers felt compelled to assign them an extra...

The poetry of dinosaurs

Behold the mighty dinosaur, Famous in prehistoric lore, Not only for his power and strength But for his intellectual length. You will observe by these remains The creature had two sets of brains -One in his head (the usual place), The other at his spinal base. Thus he could reason a priori As well as a posteriori No problem bothered him a bit He made both head and tail of it. So wise was he, so wise and solemn, Each thought filled just a spinal column. If one brain found the pressure strong It passed a few ideas along. If something slipped his forward mind Twas rescued by the one behind. And if in error he was caught He had a saving afterthought. As he thought twice before he spoke He had no judgement to revoke. Thus he could think without congestion Upon both sides of every question. Oh, gaze upon this model beast, Defunct ten million years at least.

Eurypoda Ankylosauria mass and gas

Panoplosaurus Fossils

Nodosaurids had relatively long snouts and well-muscled shoulders, reflected by the presence of a large knob of bone on the shoulder blade, the acromial process (Figure 5.20), an attachment site for the heavy shoulder musculature that characterizes nodosaurs (Figure 5.20). Nodosaurs also had flaring hips, and pillar-like limbs. Many had tall spines at the shoulder (parascapular spines). Nodosaurids are known principally from the Northern Hemisphere (North America and Europe), although new discoveries in Australia and Antarctica have extended the geographical range of these animals deep into the Southern Hemisphere. Ankylosauridae. Members of Ankylosauridae give the impression of impregnable, mobile fortresses. All are well armored (see Figure 5.14), but there are fewer tall spines along the body than in nodosaurids. The tail ends in a massive bony club, in some instances with several paired knobs or triangular spikes along its length. The head is shorter and broader in...

Behavioral Aspects Of The Huayangosaurus

Huayangosaurus Skeleton

The means by which these measures are obtained and their meanings interpreted are exceedingly important and worth describing. How, after all, do we know the size and shape of a dinosaur brain The brain is soft tissue, and commonly decomposes long before the process of fossilization can begin. As many workers have shown, however, casts can be obtained of the interior of the braincases of fossil vertebrates. Latex is painted onto the inside of a well-preserved, three-dimensional braincase (one that was not crushed during fossilization). When the latex has dried (and is flexible), it can be peeled off the inside of the braincase, and pulled through the foramen magnum ( big hole ), the opening through which the spinal cord enters the skull. The result is a three-dimensional cast of the region occupied by the brain (Figures 6.7 and 15.6). Such casts give some inkling about the shapes and sizes of brains. Dinosaur brains can be no larger than the volume of these casts, but they were...

Sauropods Of The Cretaceous Period

Vertebral Body Concavities

The presence of pneumatic concavities in the spinal bones of sauropods provided lightness without sacrificing strength. The presence of pneumatic concavities in the spinal bones of sauropods provided lightness without sacrificing strength. Traits that united the sauropods included sturdy, upright limbs to support their massive weight four or more fused or sacral vertebrae connecting the spine to the pelvic bones strong, weight-bearing feet elongation of the neck a U-shaped mouth opening optimized for stripping vegetation from stems and tree branches and pneumatic concavities in their spinal bones that provided lightness without sacrificing strength and may also have housed air sacs that were involved in these animals' breathing. Though possessing these similarities, sauropods also developed significant variation that resulted in the evolution of several distinct groups. These groups were mostly distinguished by anatomical differences of the skulls, vertebrae, and limbs.

Prehistoric Life - Graptolites

Prehistoric Arthropoda

Homalopteon, a nileid, is smooth and has much larger eyes than Asaphus or Ogygiocaris the smooth outline to the carapace may have been an adaptation for easier movement through the mud on the sea floor. The trinucleids have long genal spines extending back from the sides of their heads when the small posterior part of this animal rolled up, these spines still projected backwards and would have assisted the animal to remain stable even in soft mud. Different species of trinucleids can be distinguished by variations in the ornament round the front of the head. suggests that they were bottom dwellers they could have moved over soft sediment supported on their long genal spines. Like most trilobites they were probably deposit feeders. Brachiopods are less common than the trilobites. Apheorthis and Finckelnburgia are two orthid brachiopods which occur as single species clumps between the stromatolites. Some quiet water or deeper-shelf environments of the Middle Ordovician include many...

Allosaurus Temnonychus Claw Cutter

Broncosaurus Rex Ironclad

Target Ankles, Spines Ankylosaurus peltaspinos is a larger and more lethally adorned version of the regular ankylosaurus. With a maximum length of 40 feet, its most noticeable feature is the vast array of foot-long spines covering its carapace. The entire carapace is separated into little bony squares, and each square has a spine coming out of the center. Spines (Ex) The spines on the carapace of Ankylosaurus peltaspinos ( spiny shield ) are an extra protection. Any creature that comes into physical contact with the creature takes 1d8 points of damage, which can be avoided with a Reflex save (DC 14). Any attack made with a reach of 5 ft. or less (whether a natural attack or a melee weapon) counts as coming into physical contact. Ranged weapons and weapons with reach avoid the spines. The spine damage is a recurring attack - even if you make a Reflex save one round, you still need to make another one on the next round.

What Is An Asspect Of A Redlichia Takeoonesis Trilobites

OLENELLUS Billings, 1861 ( Fremontia Raw, 1936 Mesonacis Walcott, 1885 Paedumias Walcott, 1910), Olenellus fremonti Walcott (x7.2). Lower Cambrian of British Columbia. Bonnia-Olenellus assemblage zone in the St. Piran Sandstone (Peyto Limestone Member). (Gift of J. R. Evans to UCWM loaned by FMNH id. RLS.) The classification of the Olenellidae has undergone extensive revision in recent years (Fritz 1972 Bergstrom 1973b). Having changed generic affiliation several times, Olenellus fremonti is presently reinstated within the original denomination given by Walcott, 1910. This trilobite typically has fourteen thoracic segments. Note exaggerated development of the pleural lobe of the third thoracic segment, said to be macropleural The irregular fractute along the axis is probably due to compression. The axial spine is hollow. Juvenile form of Olenellus fremonti Walcott (xlO), from the same sample which yielded the preceding example (RLS coll., id. now at FMNH.) In (a) the specimen,...

Other Small Shelly Fossils

Hyolith Fossil

Tigraphic sections throughout the world. Protohertzina belongs to the protoconodonts, a group of spine-shaped phosphatic fossils. Interestingly, Szaniawski (1982) has shown that there are strong similarities between the microstructures of some protoconodonts and the grasping spines of modern chetognaths (pronounced KEET-o-naths) or arrowworms. Chetognaths are tiny but voracious marine predators that have sharp grasping spines near their mouths for seizing prey. Fossil chetognaths first appear in the Carboniferous. Assuming that protoconodonts and chetognaths are indeed related, Pro-tohertzina is the earliest convincing evidence for a fossil metazoan predator.

Jiangyong Zhang Fan

Gravure Danseuses Egypte Ancienne

Protopsephurus (Fig. 98) is a member of a recent family Polyodontidae (paddlefish). It was collected from Lingyuan, Liaoning, with a length of approximately 10 cm in smallest individuals and over one meter in large specimens. Judging from the matured skeletons, Protopsephurus could be the smallest in body size among the genera of Polyodontidae. This genus is the earliest fossil record of the family. An extremely long rostrum, a series of rostral splints and spine-fringed scales are the most prominent features of the family. Psephurus is the only extant polyodontid in China who lives in the drainage of the Yangtze River and littoral region of the East China Sea whereas Protopsephurus is a stem-polyodontid closely related to Paleopsephurus, a Late Cretaceous sturgeon from North America. The living paddlefish is commercially an important freshwater fish. It has naked body surface and spoon-like rostrum and is thus also called duck-mouthed sturgeon. The paddlefish eats mainly zooplankton,...

Did some dinosaurs have two brains

Take Tuojiangosaurus (twah-JEEAHNG-uh-sawr-us), or Tuojiang lizard, for example. It was named after the river in China where it was found. Tuojiangosaurus had a big bump on its spine, just over its hips. The bulge was 20 times bigger than its small brain. Today, scientists think the bump is where nerves from the back half of the dinosaur's body linked up with the spine. In an animal, the nerves connect with its brain. So many say Tuojiangosaurus had a second brain.

Coelophysoidea Neoeratosauria and Tetanurae

Derived characters include at 1, modification of the neural spines and transverse processes of the vertebrae, fusion ofthe sacral ribs with the ilium, ventral and lateral flaring of the crest above the acetabulum on the ilium, modification of the knee joint, and fusion between the upper ankle bones at 2, low ridge demarcating the maxillary antorbital fossa, spine table on axis, reduced rodlike axial spinous process, prominent acromion on the scapula, loss of digit IV phalanges, metacarpal II nearly twice the length of metacarpal I, reduced femoral trochanteric shelf, prominent extensor groove on femur, fibular condyle on proximal tibia strongly offset from cnemial crest, broadly triangular metatarsal I attached to distal part of metatarsal II.

The Great Predator of the South

What Heaven Like

belonged the order Saurischia, the suborder Theropoda, and the superfamily Allosauridae. It could measure up to 16.5 feet (5 m) in height and weigh 8.8 tons (8 metric tons). The fossil ed bones that have been found for this dinosaur include the skull, pelvis, femur, spinal column, and upper limbs. It was thought to hunt in packs, because several fossils have been found togetfler.'This made it a deadly threat to the large, herbivorous sauropods of the time.

Hanging around

The most striking feature of this powerful dinosaur is a series of spikes running down its spine. The spikes measure around 0.4 metre (1.3 feet) in length, which is big, but probably not long enough to be useful as a form of defence. Instead, they support a frill which runs down the length of the body. But what is the purpose of this crest If you can answer that, you're going to be a great palaeontologist because nobody is sure at the moment.

From Head To Tail

Dromaeosaurus Bone

Dinosaurs, like humans, belong to a group of animals called vertebrates. The key feature of all vertebrates is the spine - a stiff rod made up of small bones running from the head to the tail. The spines of dinosaurs reveal a great deal about the way they moved. In some dinosaurs, the bones of the spine were joined by flexible joints, allowing these dinosaurs to swing their necks and tails at will. In others, rodlike stiffeners made parts of the spine rigid. The rear part of the spine formed the tail, which provided The bony spikes and plates of stegosaurs were not joined to the spine, but simply set into the skin. Each type of stegosaur had spikes or plates of a distinctive shape. Perhaps these structures helped them recognize each other. Flat plates may also have regulated heat, The sauropods had the longest spines of any land animals. Diplodocus's spine - measured from the back of its head to the tip of its tail - was more than 85 ft (26 m) long. Its neck was made up of 15 bones by...


Gryposaurus Skeleton

These herbivores are also known as duckbills, because of their toothless, ducklike bills. Hundreds of self-sharpening teeth arranged in rows lined the sides of the jaws. Hadrosaurs were bipedal. They held their bodies horizontally with their stiffened tails extended for balance. There are two main groups of hadrosaurs hadrosaurines, with flat-topped skulls, and lambeo-saurines, with hollow Bony rods head crests. along spine Gryposaurus Like many hadrosaurs, Gryposaurus had a trellis of bony rods that stiffened the spine and tail. The deep tail would have been useful when swimming, and shows that hadrosaurs sometimes went into water. But they probably did this only when escaping from enemies.

Dino brains

Stegosaurus Dermal Plate

How do we know the size and shape of a dinosaur brain Casts can be obtained of the interior of the braincase. To do this, latex is painted onto the inside of a well-preserved braincase that was not crushed during fossilization. When the latex has dried (and is flexible), it can be peeled off the inside ofthe braincase, and pulled through the foramen magnum ( big hole ), the opening through which the spinal cord entered the skull in life. The result is a three-dimensional cast of the region occupied by the brain (see Figures 5.9 and 12.3 Box 12.4). Such casts give some inkling about the shapes and sizes of brains. Unfortunately, observations made of the brains of living lizards, snakes, and crocodilians show that these brains take up somewhat less room within the braincase than do those of mammals or birds. Researchers have long suspected that the brains of dinosaurs should be similarly smaller than the entire volume ofthe braincase Among fully adult individuals, it appears that there...

Order Osteostraci

At least 14 species of Boreaspis are known from sandstones laid down in the lagoons of Spitsbergen during the Early Devonian. They differ in the width of their triangular-shaped head shields, and in the length of the bony spine that grew out from the cheek area on either side. The snout was elongated in all species into a bladelike rostrum. Beside its hydrodynamic function, the rostrum was probably used to probe for prey on the muddy lagoon floor.

Nothosaur Anatomy

One primary difference between the nothosaurs and plesiosaurs was in the spinal column. Nothosaurs swam by undulating their body rather than using their paddles like the plesiosaurs. The tail was the main source of propulsion for swimming. Undulating the body to wave the tail required a flexible spine that could bend from side to side.


Lapworthella Meaning

The individual spines of a sponge's skeleton could be considered sclerites, except that many of these spines visible at the surface of the sponge are deeply embedded in the body of the sponge, and the word spicule is preferable for skeletal elements of this sort. Sponge spicules support the soft tissue that forms the filter feeding chamber, In addition to the phosphatic sclerites, a number of calcium carbonate Cambrian shelly fossils are now interpreted as representing isolated sclerites. The sclerite Chancelloria is very commonly found in Early Cambrian acid residues. A natural association of Chancelloria sclerites on a bedding plane surface is shown in figure 4.10. Each sclerite is shaped something like the hub and spokes of a wagon wheel, except that the spokes often curve away from the hub instead of radiating straight and in the same plane. Six hollow spines radiate out from the central boss, and sometimes a seventh spine projects outward, perpendicular to the bases of the other...

Ichthyosaur Anatomy

The spine of ichthyosaurs has been described as a row of doubly concave vertebrae the shape of hockey pucks. This unusual design differs from that of most terrestrial reptiles in that it favors locomotion through the water by use of the tail. The ichthyosaurs propelled themselves by waving their large tails from side to side. The hockey--puck design gave the back great flexibility and strength. The earliest known ichthyosaurs from the Triassic Period did not yet have this kind of spine. Their spine was made up of smaller, longer bones that allowed the animal to swim by wriggling its entire body from side to side, like an eel.

Internal Structure

Snakes are scaly reptiles with long bodies and no legs. Some are poisonous, but others are not. Like all reptiles, they have a spinal column and a skeletal structure composed of a system of vertebrae. The anatomical differences between species reveal information about their habitats and diets climbing snakes are long and thin, burrowing snakes are shorter and thicker, and sea snakes have flat tails that they use as fins.

Fish and chips

Bird Occipital Bone

At the front end of the vertebral column of chordates are the bones of the head, composed, as we have seen, of the skull and mandible (Figure 4.6). Primitively, the skull has a distinctive arrangement the braincase, a bone-covered box containing the brain, is located centrally and toward the back of the skull. At the back of the braincase is the occipital condyle, the knob of bone that connects the braincase (and hence the skull) to the vertebral column. A rear-facing opening in the braincase, the foramen magnum, allows the spinal cord to attach to the brain. Located on each side of the braincase are openings for the stapes, the bone that transmits sound from the tympanic membrane (ear drum) to the brain. Finally, covering the braincase and forming much of the upper rear part of the skull is a curved sheet of interlocking bones, the skull roof (inset to Figure 4.6).


Reptiles are vertebrates, meaning that they are animals with a spinal column. Their skin is hard, dry, and flaky. Like birds, most reptiles are born from eggs deposited on land. The offspring hatch fully formed without passing through a larval stage The first reptiles appeared during the height of the Carboniferous Period in the Paleozoic Era. During the Mesozoic Era, they evolved and flourished, which is why this period is also known as the age of reptiles. Only 5 of the 23 orders that existed then have living representatives today.


Thamnopora Environment

E acanthodian spine (Vertebrata Osteichthyes) e acanthodian spine (Vertebrata Osteichthyes) Cephalaspis is another armoured fish with fixed lateral fins which probably gave slightly better stability. This fish is characterized by large sensory areas on the headshield and high-placed eyes. Our reconstruction shows a Turonia in the water above, its body covered with minute denticles. It is depicted as dead, drifting downstream, slightly buoyed by gases from its decomposition. The skeleton would soon rupture and the denticles scatter and be incorporated into the sand and mud. Acanthodian spines can also be found in this environment. The acanthodians are small spiny fish which, though they had jaws, were generally edentulous. Many were microphagous, swimming in deeper v ater and probably not competing with the agnathids. Most of the faunas were suspension feeders. Brachiopods are common, particularly spiriferides. Some are winged, and have large spiral lophophore supports, and seem well...


Laudonia, a Lower Cambrian trilobite with conspicuous me-tagenal spines. Greatest width 3 cm. (From M. McMenamin 1989) FIGURE 4.15. Laudonia, a Lower Cambrian trilobite with conspicuous me-tagenal spines. Greatest width 3 cm. (From M. McMenamin 1989) FIGURE 4.13. Cephalon of the Lower Cambrian olenellid trilobite opposite above), judomia. Note its large eyes and long genal spines. Width of specimen (spine tip to spine tip) 3.6 cm. (From M. McMenamin 1988) The most bizarre of the Early Cambrian echinoderms was Helicoplacus, a spiralled, spindle-shaped fossil restricted to Lower Cambrian sediments of western North America (figure 4.18). The whole test could be expanded and contracted expansion apparently was accomplished by inflation from the inside. In one species, there are spines on some of the plates. Helicoplacus had one or more food grooves like the eocrinoids, except that instead of being on the arms, the grooves wound around the body of the animal, following the...


Guppy Clipart Com

The vertebral column is composed of distinct, repeated structures (the vertebrae), which consist of a lower spool (the centrum), above which, in a groove, lies the spinal cord (Figure 4.5). Planted on the centrum and straddling the spinal cord is the neural arch. Various processes, that is parts of bone that are commonly ridge-, knob-, or blade-shaped,


The most spectacular of Stromer's dinosaur discoveries was Spi-nosaurus ( spine lizard ), an incredibly large predator that is estimated to have been about 50 to 57 feet (15 to 17 m) long. An unusual feature of Spinosaurus were long spines on its back that probably formed a large, sail-like structure. The spines alone were up to 6.6 feet (2 m) tall, adding height to a creature that must have stood about 16 to 20 feet (5 to 6 m) tall at the hips. Of the creature's skull, Stromer found only the lower jaws and a fragment of the upper, but these fossils, too, revealed an unusual trait. Whereas most thero-pods had bladelike teeth, the teeth of Spinosaurus were conical and unserrated, more like those of a crocodile. Although the fossil-hunting team led by Josh Smith was unable to find a significant new specimen of Spinosaurus, its efforts have helped reinstate the gigantic theropod as a legitimate contender for the title of largest known predatory dinosaur. Nonetheless, additional hard...

Individual Bones

Tyrannosaurus Bones

The neck bones of T. rex have big prongs on them, called neural spines. These are the attachment points for huge muscles that link up on the other end to the top of T. rex's head. The small size of the neck bones compared with the massive head they supported suggests that T. rex must have had massive neck muscles. What other benefit would those neck muscles have provided Feel the back of your neck when you're biting down and pulling on a piece of taffy. Those muscles on a T. rex would have been useful for yanking at food. Below the prong on each vertebra of T. rex and other dinosaurs is the rest of the neural arch. Pairs of big facets that rise vertically above the spinal cord on the front and back of each arch are called zygapophyses. They are the points at which each back vertebra is linked to the next. The space between the large zygapophyses of adjacent vertebrae on T. rex's neck helped make its neck very flexible.

Sexual Dimorphism

The basal ornithopod Hypsilophodon shows variation in the spine that may denote an anatomical difference between males and females. In this case, the number of vertebrae fused to the pelvis, known as sacrals, may be either five or six, and varies from individual to individual. This is a highly unusual occurrence because the number of sacral vertebrae is usually the same for all members of a taxon. It has been suggested that this is the result of sexual dimorphism.


Cambrian Prehistoric Life Pictures

Zacanthoides typicalis Walcott (xll). Chisholm Shale, M. Cambrian, Half Moon Mine, Pioche, Nevada. (Specimen collected by Afton Fawcett. RLS coll., id., now at FMNH.) Once again it is worth showing the comparison berween standard photography of the specimen in air (a) and what can be seen with xylene immersion (b). The long axial spine originates from the eigth thoracic axial ring. Developmental stages (late meraspid degrees) in the Middle Cambrian trilobite Paradoxides gracilis (Boeck), M. Cambrian, from Jince (Jinetz), Bohemia. The specimen in (a) (x8.6) exhibits much elongated genal spines as well as pleural spines of the second thoracic segment. The latter character is common to meraspid stages of other paradoxidids (see plate 92) and is reminiscent of the long pleural spines associated with the third segment macropleurae observed

The Cephalon

Paradoxides Trilobite With Naming

The outline of the cephalon may be semicircular to ogival in its anterior portion, straight or gently curved at the posterior margin, where articulation with the first thoracic segment occurs. The lateral and anterior margin of the cephalon are inflected into the doublure, a narrow strip of the ventral side of the exoskeleton that is mineralized like the dorsal side. Attached to the anterior part of the doublure is a small shield called hypostoma, one of the few hard parts to be found on the underside of the trilobite. The angle between the backward-sloping lateral margin of the cephalon and the posterior margin is called the genal angle. This termination can be rounded or, as in our example in plate 1, prolonged into long genal spines. The axial lobe extends into the cephalon, where it takes the name of glabella. This can be a very convex bulging region, sometimes extending all the way to the anterior margin, or it can terminate earlier, defining a flat preglabellar field. The...

The Pygidium

Extreme variation in size occur for this portion of the carapace, and the extension of the pygidium seems related to the number of thoracic segments. In the example in figure 2, the pygidium is small-the trilobite is called micropygous-but the number of thoracic segments is relatively large (twenty for the example shown). Frequently, the pygidium reaches a size comparable to that of the cephalon isopygous trilobites), as will be seen from many examples in the atlas. In such cases, the number of thoracic segments is usually small. Macropygous trilobites are provided with pygidium larger than the cephalon. The axial lobe clearly extends into the pygidial region in a great majority of trilobite species, so that the trilobation is usually preserved. It may extend to the posterior margin or terminate earlier. Evidence of the segmentation from which the pygidium was derived is to be found in the frequently observed ribbing of the axial lobe, resembling the articulated axial...

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