84 PALEONTOLOGY 



6. Tellinacea. Sinu-palliate. "Ornament concentric 

 striate. 



Tellina (Fig. 24, b), one of the commonest of seaside 

 shells, dates from the Jurassic. Donax (Eoc.-Rec.) is 

 opisthogyral, and has a short posterior side (like Nucnla). 



B. WITH TEETH OF CYRENOID TYPE : * ' ^ ' 7 



2a, b, 40 



1. Cyprinacea. Concentric ornament, rarely more 

 than striate; valve-margins smooth. Umbones anterior. 



Various early Jurassic forms, too rarely found with 

 hinge decipherable, have been shown by Prof. Douville 

 to be the originators of this type of hinge. Better known 

 are Cyprina (Fig. 24, e), a sub-circular shell with beaks 

 curved forward, crowding out the anterior cardinal tooth, 

 abundant in the colder seas (Jur.-Rec.) ; Trapezium (Cypri* 

 cardia), a somewhat four-sided form (Jur.-Rec.); and 

 Isocavdia (Mio.-Rec.), with highly spiral umbones. 



2. Astartacea. Concentric (striate or costate) orna- 

 ment, never radial ; valve-margins usually crenulate. 

 Ligament external (except in Crassatella.) Integri- 

 palliate. Trias. -Recent. 



The chief genera are the nearly circular Astarte 

 (Fig. 23, e) with numerous sub-genera, the trapezoidal 

 Opts (Mesozoic) and Crassatella (Cret.-Rec.) 



3. Cyrenacea. Fresh-water habitat. Concentric 

 striate ornament ; valve-margins smooth ; integri- or 

 sinu-palliate. Umbones fairly central. 



Anterior and posterior lateral teeth well developed. 

 The chief genera are Cyvena (Jur.-Rec.) and Covbicula 

 (Eoc.-Rec.), already described. 



4. Carditacea. Radial ornament; valve -margins 

 crenulate ; umbones anterior, leading to a* lengthening 

 of the teeth 2b and 36. The chief genera are Cardita 

 (Jur.-Rec.) and Venericardia (Fig. 24, /, Cret.-Rec.), the 



