Phylum Mollusca

Animals with a body composed of a head and/or a foot, a visceral mass containing the internal organs, and a sheet of tissue (the mantle) which can secrete a calcareous shell. A space between the mantle and the visceral mass, known as the mantle cavity, contains gills and acts as a respiratory chamber. Most molluscs have a free-swimming larval stage.

class MONOPLACOPHORA (Cambrian to Present)

Mollusca with a single valve containing several paired muscle scars. Shallow marine benthos in the Palaeozoic; living at abyssal depths today.

class POLYPLACOPHORA (Cambrian to Present)

Marine mollusca, normally with a row of calcareous plates; commonly called the chitons.

class APLACOPHORA (Present)

Worm-like animals without shelly plates.

class SCAPHOPODA (Devonian to Present)

Benthic, marine mollusca with an external tubular tapering curved shell, open at both ends. They live partly embedded in the sediment, with the narrow end of the shell extending up into the water. Scaphopods feed on micro-organisms and organic debris.

class GASTROPODA (Cambrian to Present)

Mollusca usually with a single valve, and no internal septa. Most gastropods crawl on their foot, but in some the foot is adapted for swimming. Gastropods may be marine, terrestrial, or freshwater.

subclass PROSOBRANCHIA (Cambrian to Present)

Gastropods with mantle cavity opening to front. Shell rarely absent.

order Archaeogastropoda (Cambrian to Present)

Mostly with paired gills, each gill having two rows of filaments. Mainly algal feeders.

order Mesogastropoda (Ordovician to Present)

Usually with a single gill having one row of filaments. Often with a siphon,, Include benthic grazers, suspension feeders and carnivores; some are pelagic.

order Neogastropoda (Cretaceous to Present)

Gastropods with an inhalent siphon emerging through a siphonal canal on the aperture margin. Mainly carnivorous; radula with few teeth (usually not more than three) in a row.

subclass OPISTHOBRANCHIA (Carboniferous to Present)

Mantle cavity opens at side or rear of body. Shell often reduced or absent,. Includes benthic forms and the pelagic Pteropods.

subclass PULMONATA (Jurassic to Present)

No normal gills; the mantle cavity acts as a lung. Mostly terrestrial; a secondary gill occurs in some freshwater forms.

class CEPHALOPODA (Cambrian to Present)

Marine mollusca with a single-valved shell partitioned by septa, which are traversed by a fleshy tube, the siphuncle. All known modern cephalopods are carnivorous.

subclass NAUTILOIDEA (Cambrian to Present)

Siphuncle variable in position, but not inflated between septa. No closely packed calcareous cones within siphuncle. Curvature of shell variable. Septal sutures with smooth curves.

subclass ENDOCERATOIDEA (Ordovician and ? Silurian)

Posterior part of siphuncle filled by close packed plates. Straight or slightly curved shell. Septal sutures straight or slightly flexured.

subclass ACTINOCERATOIDEA (Ordovician to Carboniferous)

Straight shells, with siphuncle inflated between septa. Other internal structures may also be present. Septal sutures straight or slightly flexured.

subclass BACTRITOIDEA (Ordovician to Permian)

Straight or gently curved shells, with siphuncle in contact with ventral wall. Septal suture with a strong V-shaped lobe.

subclass AMMONOIDEA (Devonian to Cretaceous)

Cephalopods with angular or more complex septal sutures. Normally with ventral siphuncle and planospiral shell. Includes the goniatites (Palaeozoic), ceratites (Triassic) and ammonites (Jurassic and cretaceous).

subclass COLEOIDEA (Devonian to Present)

Most fossil coleoids are belemnites with a cigar-shaped calcite guard, which contains a conical phragmacone (with septa and ventral siphuncle) at one end. The belemnites became extinct in the early Tertiary (Hancock, 1967). The modern coleoids (cuttlefish, squids and octopus) have no guard.

class BIVALVIA (Cambrian to Present)

Marine or freshwater molluscs with two calcareous valves joined at the hinge with a flexible ligament. Bivalves have very varied modes of life; they include fixed forms (by byssus or cementation) and mobile forms (free-living, sessile, burrowing and boring); they include filter feeders and detritus feeders.

subclass PALAEOTAXODONTA (Ordovician to Present)

The nuculoids, with simple gills and many small teeth (taxodont) are mostly shallow burrowers.

subclass CRYPTODONTA (Cambrian to Ordovician to Present) Toothless burrowers with simple gills.

subclass PTERIOMORPHIA (Ordovician to Present)

This large group includes bivalves with more complex gills. They are varied, but the fossil record suggests that they may have a common origin.

order Arcoida (Ordovician to Present)

Forms with both adductor muscles equal (isomyarian), and with a characteristic area between the hinge line and the umbo. Usually with many small teeth.

order Mytiloida (Devonian to Present)

The mussels are usually byssally attached and have a reduced anterior adductor muscle (heteromyarian).

order Pterioida (Ordovician to Present)

Although descended from byssally fixed forms (and thus hetero-myarian or monomyarian), many pterioids have developed other modes of life including the free-living pectinids and the cemented oysters.

subclass PALAEOHETERODONTA (Cambrian to Present)

This group includes the earliest known bivalves (Middle Cambrian). Most forms have the adductor muscles about the same size (iso-myarian), and are equivalved.

order Modiomorphoida (Cambrian to Present)

Includes the actinodonts, which have a few elongate teeth radiating from the umbo; they may be ancestral to most other bivalve groups.

order Unionoida (Devonian to Present)

Freshwater bivalves, smooth except for growth lines. Characterized by a thick outer coating of horny material (periostracum) covering the calcareous part of the shell.

order Trigonoida (Devonian to Present)

Marine bivalves with an angular posterior margin, and a few strong teeth near the umbo.

subclass HETERODONTA (Ordovician to Present)

The name refers to the fact that the teeth may include a group near the umbo (cardinal teeth) as well as lateral teeth. The adductor muscles are usually equal in size.

order Veneroida (Ordovician to Present)

Active, nestling or burrowing heterodonts. Teeth well developed.

order Myoida (Carboniferous to Present)

Burrowing heterodonts with siphons, weak teeth and thin shell.

order Hippuritoida (Silurian to Cretaceous)

Thick-shelled bivalves, mainly attached by one valve. This group includes equivalved early forms, but the later members include the rudists, which have one large fixed conical valve and a small free valve on top. The rudists formed extensive reefs in low latitudes during the Cretaceous.

subclass ANOMALODESMATA (Ordovician to Present)

Burrowing, byssate or cemented bivalves with weak teeth (and never any lateral teeth).

class ROSTROCONCHIA (Ordovician to Permian)

order Conocardiacea (Ordovician to Permian)

A peculiar group of mollusca in which the two valves have become fused.

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