378 



MOLLUSCOIDEA 



PHYLUM V 



Fig. 556. 



A, Orhiculoifka eircc Bill. Ordovician; Belle- 

 \'ille, Canada. Ventral valve, i/i (after Billings). 

 B, 0. nitida Pliill. Lower Carboniferous ; Mis- 

 souri. X, Dorsal; y, Ventral valve, Vi- 



or flattened. Dorsal valve larger, usually depressed conical. Pedicle furrow 



E originating behind the apex, extending 



over a greater or lesser portion of the 

 radius of the valve, and produced at the 

 distal end into a short tubular sipho, emerg- 

 ing on the interior surface near the posterior 

 margin. Surface with fine, crowded or dis- 

 tant, rarely lamellose, concentric lines, 

 occasionally crossed by radiating lines. 

 Ordovician to Cretaceous; North and South 

 America, Europe, and probably elsewhere. 



Discina Lamarck. Very much like 

 OrUculoidea, but the pedicle emerges through 

 the ventral valve antero- posteriorly, im- 

 mediately beneath the beak, instead of through a sipho postero-anteriorly as 

 in that genus. Recent. 



Until recently Discina embraced all fossil Discinoid shells, but at present 

 this genus seems to be restricted to a single species, 

 D. striata, living off Cape Palmas, West Africa. 



Discinisca Dall (Fig. 557). Like Orhiculoidea, 

 but with a small septum, as in Discina, behind 

 which is an impressed area, externally concave and 

 internally elevated. This is perforated by a longi- 

 tudinal fissure, extending from a short distance 

 behind the septum nearly to the posterior margin. 

 Tertiary to Recent ; North America and Europe. 



Pelagodiscus Dall. Like Discinisca, but the brachia are without spirals. 

 Recent ; deep oceans. 



ScJiizotreta Kutorga. Ordovician and Silurian ; Russia and North America. 

 Lindstroemella and Roemerella Hall and Clarke, are genera related to Orhi- 

 culoidea. Devonian ; North America. 



Fig. 557. 



Discinisca lamellosa (Brod.). Recent ; 

 Peru. A, Side-view. B, Interior of 

 ventral valve. C, Exterior of same. 



Superfamily 4. CRANIACEA Waagen. 



Specialised, cemented calcareous Neotremata without pedicle or anal openings at 

 maturity. Pedicle functional prohcddy only during nepionic growth. Ordovician 

 to Recent. 



Family 1. Craniidae King. 



Crania Retzius. Shell inequivalve, sub-circular in outline. The interior 

 of both valves shows two pairs of large adductor scars, the posterior 

 of which are widely separated and often strongly elevated on a central 

 callosity. Impressions of the pallial genital canals coarsely digitate. Ordo- 

 vician to Recent ; maximum development in Ordovician and Cretaceous. 



Pseudometoptoma von Huene. Very large thick-shelled forms with high 

 dorsal valves. Ordovician ; Esthonia. 



Philhedra Koken. Ordovician ; Europe and America. 



Petrocrania Raymond {Craniella (Ehlert 1888, non Schmidt 1870). Large 

 Craniids with S-shaped vascular impressions. (?) Ordovician and Devonian ; 

 North America and Europe. 



