PELECYPODA—PRIONODESMACEA. 



417 



Fig. 540. Ischyrodonta truncata. (After Ulrich, Pal. Ohio, VII.) 



XLIII. Arca Lamarck. 



Shell thick, ventricose, trapezoidal. Beaks conspicuous, distant, 

 separated by an area which has many ligamental grooves con- 

 verging from the hinge margins to the beaks. 

 Surface radially sculptured. Hinge line wide, 

 straight, transversely dentate ; teeth small 

 and similar (taxodont). Cretacic-Recent. 



This has been divided into several subgen- 

 era of which Barbatia is one. 

 146. A. quindecemradiata Gabb. (Fig. 541.) 



Fig. 541. Area quin- 

 decemradiata. Internal 

 mold. (After Whitfield, 

 Pal. N. J., I.) 



Cretacic. 



Rather small, ventricose and very inequilateral ; anterior round- 

 ing rapidly into basal margin ; beaks slightly incurved, a little in 

 front of the mid-length of the hinge line ; obliquely truncate 

 posteriorly ; surface with coarse rounded ribs. 



Jerseyan (Vin cento wn) of New Jersey. 



XLIIIa. Barbatia (Gray) Adams. 

 Central teeth smallest and vertical ; lateral teeth becoming 

 gradually larger, more distant, and more oblique. Periostracum 

 hairy. Cretacic-Recent. 



147. B. micronema (Meek). (Fig. 542.) Cretacic. 

 Growth and radiating lines faint. Beaks in anterior fourth. 

 Colorado Group of Wyoming, Utah, Texas. Dakota of Texas. 



148. B. cuculloides (Conrad). Eocenic-Oligocenic. 

 Valves marked with strong, subcentral sinus. Posterior side 



elongated, strongly ribbed, and carinated ; anterior end truncate. 

 Surface reticulate by strong growth lines passing over the 

 radiating ribs. 



Eocenic : Georgia, Alabama, Mississippi, Arkansas. Lower 

 Oligocenic : Mississippi. 



