PELECYPODA—TELEODESMACEA. 



537 



Differs from A. vera in the less prominent and more nearly cen- 

 tral beaks and the more numerous and regular concentric ridges. 

 Coal Measures : Ohio, Arkansas, Oklahoma. 



CXXXV. Arctica Schumacher. (Cyprina Lamarck.) 

 Orbicular or oval, inflated, with thick epidermis. Beaks prom- 

 inent, incurved. Lunule absent. Three cardinal and one ridge- 

 like posterior lateral in each valve ; the middle cardinal of left 

 valve the largest. Surface marked with concentric striae. Margins 

 of valves smooth. Ligament external and prominent. Pallial line 

 simple. Jurassic-Recent. 

 490. A. coteroi Castillo and Aguilera. (Fig. 733.) Jurassic. 



Fig. 733. Arctica coteroi, left and front views of internal mold. (After Cragin.) 



Surface of shell when preserved marked with concentric growth 

 lines and occasional concentric grooves. Length less than 2 

 inches. 



Upper Jurassic of Texas (Malone) and central Mexico. 

 491. A. occidentalis (Whiteaves). (Fig. 734.) Comanchic. 



Fig. 734. Arctica occidentalis, right valve. (After Stanton. 



