PLATE

ALOKISTOCARE Lorenz, 1906. Alokistocare harrisi Robison (x6.5). Wheeler Shale, M. Cambrian, Antelope Springs, Utah. (Collected by Afton Fawcett. RLS coll., id., now at FMNH.) Following Opik (1961), the family Alokistocaridae Resser, 1939, is regarded as a younger synonym of Papyriaspididae Whitehouse, 1939. Specimen whitened with magnesium oxide.

Alokistocare piochensis (Walc o tt) (x4. 0). Chisholm Shale, M. Cambrian, Half Moon Mine at Pioche, Nevada. (Collected by Afton Fawcett. RLS coll., id., now at FMNH.) One of the free cheeks is displaced.

PIOCHASPIS, n.g. Piochaspis sellata, n. sp., holotype. Chisholm Shale, M. Cambrian, Half Moon Mine at Pioche, Nevada (xl4.4). (Collected by Afton Fawcett. RLS coll., id.) Plate 99 (a) shows the specimen whitened with magnesium oxide; plate 99 (b) shows the same specimen while immersed in xylene, revealing additional details. A

morphometric study by the author of the population of ptychoparid trilobites from this locality (unpublished, but discussed with F. Rasetti and A. Palmer) shows this trilobite to be distinct from Alokistocare piochensis (Walcott) and Alokistocare packi (Resser) from the same beds. The description of this new trilobite is the following: Anterior border convex, anterior border furrow with median inbend, preglabellar field depressed mesially, elevated laterally; glabella tapering forward, rounded in

from, with three pairs of shallow lateral furrows; axial furrows deep, facial sutures diverging in front of eyes; eye ridges narrow, - 0.3 of glabellar width at midlength, palpebral areas convex, narrow anteriorly, posterior area of fixigena deeply furrowed. Librigenae with pointed genal spines, flaring posteriorly at - 45°. Thorax of seventeen segments; pleurae grooved, pleural spines short, outward pointing. Pygidium small, devoid of border, with prominent axis, one axial ring. The entire collection of 67 complete individuals relating to this study is now at Field Museum of Natural History. FMNH record numbers for the various groups are: Piochaspis sellata, holotype, PE 54115; Piochaspis sellata, 48 syntypes, PE 5411654163; Alokistocare packi, 10 specimens, PE 54164-54173; Alokistocare piocbensis, 8 specimens, PE 54174-54181.

ELRATHIA Walcott, 1924. Elrathia kingii (Meek) (xl.8). Wheeler Shale, Antelope Springs, Millard County, Utah. (Collected by Aiton Fawcett. RLS coll., now at FMNH.) In this fossilization, the trilobite exoskeleton is replaced by calcite; the crystals of this mineral extend in a so-called "cone-in-cone" structure well beneath the original carapace.

Elrathia kingi (M e e k) (x3.5). Same origin as for previous plate. (Collected by Afton Fawcett. RLS coll., now .it l-'MNI I.) Specimen of approximately maximum size for this species.

Elrathia kingii (Meek) in association with the agnostid Peronopsis interstricta (White) (x.8). (Collected by Afton Fawcett. RLS coll., now at FMNH.) Various stages of growth of Elrathia are represented in this unusual plate.

Family Ptychopariidae Matthew, 1887

Subfamily Ptychoparnnae Matthew, 1887

PTYCHOPARIA Hawle and Corda, 1847. Ptych op aria striata (Emmrich), M. Cambrian, Jince Formation, Jince, Bohemia (x3.0). This genus is thought to be the progenitor of a very broad group of trilobites. In this example, two posterior thoracic segments and the pygidium are missing (or enrolled underneath). The striated surface of the cranidium, origin of the adjectival name of this trilobite, is evident in this photograph. (RLS coll.)

Family Asaphiscidae Raymond, 1924

Subfamily Asaphiscinae Raymond, 1924

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