PELECYPODA—TELEODESMACEA. 563 



shaped posteriorly and overlaps a groove which runs backward 

 from the beak. 



Maryland, Massachusetts ? 



CLXII. Venus Linne. 

 Valves equal, inequilateral, oval or rounded, heart-shaped, thick, 

 concentrically or radiately sculptured. Margins crenulate. Hinge 

 plate broad, with three cardinal teeth in each valve and no lateral 



Fig. 772. Venus mercenaria, right and left valves, X Yz- (Md. Surv. ) 



teeth. Pallial sinus short, angular. The two adductor scars sub- 

 marginal. Jurassic-Recent. 



559. V.mercenaria Linne. (Fig. 772.) Miocenic-Recent. 

 Type of genus. 



Atlantic coast region. 



CLXIII. Meretrix Lamarck. 



Thick, trigonal, smooth or concentrically striated. Lunule 

 well marked. Margin entire. Ligament external. Hinge thick, 

 with three cardinal teeth in each valve, two laterals in the right 

 and one in the left, beneath the lunule ; no posterior laterals present. 

 Pallial sinus rather shallow, not acutely angular. 



Differs from Venus in the possession of lateral teeth, and in 

 lacking the fine crenulations of the valve margin. Jurassic-Recent. 



560. M. tippana Conrad. (Fig. 773.) Cretacic. 

 Beaks slightly anterior to center. Length slightly exceeding 



height. Concentric lines fine and regular. 



Ripleyan of New Jersey (Cliffwood-Wenonah), Arkansas, Mis- 

 sissippi, Texas. Also Cretacic of Mexico. 



