220 MOLLUSCA. LAMELLIBRANCHIA 



Opis. Shell trigonal, cordiform, convex, with an oblique keel ex- 

 tending from the umbo to the posterior border. Umbones prominent, 

 incurved or sub-spiral. Lunule large and very deep. Surface gene- 

 rally with concentric furrows. One cardinal tooth in the right valve, 

 two in the left. Pallial line simple. Trias to Chalk. Ex. 0. lunu- 

 latus, Inferior Oolite. 



Crassatellites ( = Crassatella). Shell solid, oblong or sub- 

 trigonal, attenuated behind. Surface smooth or concentrically 

 furrowed. Margins of valves smooth or crenulated. Umbones 

 small, close together. Lunule distinct. Ligament internal, placed 

 in a pit under the umbo. Hinge with two (sometimes three) cardinal 

 teeth, and some small laterals. Adductor impressions deep. Pallial 

 line simple. Cretaceous to present day. Ex. C. sulcatus, Barton 

 Beds. 



Cyrena. Shell cordiform, oval, or trigonal, usually with con- 

 centric ridges ; umbones often corroded. Hinge with three cardinal 

 teeth ; one anterior and one posterior lateral in the left valve, and 

 two of each in the right valve. Ligament prominent, external. 

 Pallial line usually entire. Lias to present day. Lives in fresh and 

 brackish water. Ex. C. ceylanica, Recent. 



Corbicula. Similar to Cyrena, but with the lateral teeth 

 lamellar and transversely striated. Eocene to present day. Fresh 

 water. Ex. C. Jlumi?ialis, Pliocene to present day. 



Cardita. Shell oval or oblong, elongated, very inequilateral, 

 with prominent scaly ribs ; often a little gaping and sinuous at its 

 ventral margin. Umbones prominent, anterior. Lunule present. 

 Ligament external. In the right valve two long, parallel cardinal 

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

 short anterior cardinal, and one long posterior cardinal, and a 

 rudimentary posterior lateral tooth. Adductor impressions large. 

 Pallial line simple. Trias to present day. Ex. C. calyculata, 

 Recent. 



Venericardia. Shell oval, triangular, or heart-shaped, 

 inequilateral, with radiating ribs. Umbones prominent. Ventral 

 margin crenulated internally, not sinuous. Ligament external. 

 Hinge-plate thick ; in the right valve two oblique cardinal teeth and 

 one small or rudimentary anterior lateral; in the left two diverging 



