PELECYPODA—PRIONODESMA CEA . 40 3 



101. Y. longifrons (Conrad). (Fig. 516.) Cretacic. 



Fig. 516. Yoldia longifrons; with enlargement of hinge. (N. J. Pal., I.) 



Surface of shell polished, marked only by fine concentric growth 

 lines. 



Ripleyan of New Jersey (Woodbury), Mississippi, Arkansas. 



102. Y. scitula Meek and Hayden. (Fig. 515.) Cretacic. 

 Smaller than Y. longifrons (length about -| inch) and marked 



with distinct and regular concentric lines, which are strong on the 

 middle of the valves. 



Fox Hills : South Dakota, Montana. Pierre : Montana. Mon- 

 tanan : Assiniboia. 



XXXI. Parallelodon Meek and Hayden. 

 Elongate to subquadrate. Anterior end angular at cardinal line 

 and rounded below. Posterior end rounded or obliquely subtrun- 

 cate. Beaks anterior to middle. Hinge line long and straight. 

 Concentric striae often lamellose. Hinge with several small, diverg- 

 ing teeth beneath the beak and two to four long, lateral teeth 

 near the posterior extremity, Ligament external. Pallial line 

 simple. Devonic-Tertiary. 



103. P. chemungensis Hall. (Fig. 517, a.) Devonic. 



Fig. 517. a, Parallelodon chemungensis, left valve ; b, c, P. hamiltonice % right 

 valve and internal mold with valves in conjunction. (Pal. N. Y., V.) 



Distinguished by its narrow form, truncated posterior end, and 

 anterior constriction of the base. Concentric lines irregular and 

 lamellose. 



Chemung : New York, Pennsylvania. 



104. P. hamiltoniae Hall. (Fig. 517, b, c.) Devonic-Mississippic. 

 Differs from P. chemungensis in its broadly rounded posterior 



