378 



MOLLUSCOIDEA 



PHYLUM V 



Fig. 556. 



A, OrUculoidca circe Bill. Ordovician; Belle' 

 ville, Canada. Ventral valve, Vi (after Billings). 

 B, 0. nitida Phill " - 



souri 



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

 B originating behind the apex, extending 



over a greater or lesser portion of the 

 radius of the valve, and prodiiced at the 

 distal end into a short tubulär sipho, emerg- 

 ing on the interior surface near the posterior 

 margiii. 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 

 Do™-' 2/,^vertra?vXM/r' ' ^''' Ofbiculoidea, 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 Dali (Fig. 557). Like Orbiculoidea, 

 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. ^.^l^;,^^ 

 Tertiary to Recent ; North America and Europe. 



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

 Recent ; deep oceans. 



Schizotreta Kutorga. Ordovician and Silurian ; Russia and North America 

 Lindstroemella and Boemerella Hall and Clarke, are genera related to Orhi 

 culoidea. Devonian ; North America. 



Fig. 557. 



Discinisca lamellosa (Brod.). Recent ; 

 A, Side-view. B, Interior of 

 valve. C, Exterior of same. 



Superfamily 4. CRANIACEA Waagen. 



Specialised, cemented calcareous Neotremata without pedicle or anal openings at 

 maturity. Pedicle functional probably only during nepionic growtli. Ordovician 

 to Recent. 



Family 1. Oraniidae King. 



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

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

 of vi^hich 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 Gi^hlert 1888, no?i Schmidt 1870). Large 

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

 North America and Europe. 



