MOLLUSCA. LAMELLIBRANCHIATA. 143 



Section 1. Integripalliata. 



Siphons short, not retractile. Pallial line without 

 sinus. 



Cardita. Shell oblong, elongated, very inequilateral, 

 ornamented with prominent radiating ribs. Often a little 

 gaping and sinuous at its ventral margin. Lunule present. 

 Ligament external. In the right valve two cardinal teeth 

 and a small anterior-lateral tooth. In the left valve one 

 cardinal and a posterior elongated lateral tooth. Adductor 

 impressions very large. Pallial line simple. Trias to 

 present day. 



Astarte. Shell solid, inequilateral, compressed, closed, 

 more or less trigonal or sub-orbicular. Surface smooth or 

 with concentric furrows or striae. A thick epidermis. 

 Umbones prominent. Lunule distinct. Escutcheon elon- 

 gated. Ligament external. Two cardinal teeth in each 

 valve, lateral teeth rudimentary. Adductor impressions 

 strongly marked, above the anterior one is a pedal im- 

 pression. Pallial- line simple. Silurian to present day. 

 (Those in the Palaeozoic doubtful.) 



Opis. Shell trigonal, cordiform, convex, with an 

 oblique keel extending from the umbone to the posterior 

 border. Umbones prominent, incurved, or sub-spiral. 

 Lunule large. Surface generally with concentric furrows. 

 A cardinal tooth in each valve. Ligament external. 

 Adductor impressions very deep. Pallial line simple. 

 Trias to Chalk. 



Crassatella. Shell solid, oblong or sub-trigonal, 

 attenuated behind. Surface smooth or concentrically 

 furrowed. Margins of valves smooth or crenulated. Um- 



