MEDINA SANDSTONE, 



u 



392. 18. MODIOLOPSIS? PRIMIGENIUS. 



Unio primi genius. Conkad, Annual Report 1838, p. 113; 1839, p. 66. 

 Cypricardia alata. Hall,, Geological Report 4th District, pag. 48, fig. 3. 



Subrhomboidal, rounded anteriorly, expanded and alate posteriorly 3 hinge- line straight, 

 longer than the length of the shell ; anterior extremity short ; ventral margin rounded ; surface 

 marked by strong concentric imbricating lines, which are crossed by fine radiating striaj on the 

 posterior half of the shell. 



The casts of this shell usually preserve only some remains of the concentric lines, and these 

 are sometimes obliterated. In the casts the hinge-line is shorter than the length of the shell, and 

 presents on the anterior slope a fold with a depression on either side, like the impressions pro- 

 duced by the teeth of the shell. In addition to this, there is a single fold running nearly the 

 whole length of the hinge-line, exhibiting in the left valve a well marked depression. The 

 surface markings present various degrees of perfection ; and the differences between the entire 

 cast and the specimens in different conditions is so great, as to induce a belief that more than 

 one species exists ; still I have been unable to distinguish characters in the specimens I have, 

 which warrant a separation. 



Fig. 2 a. A small specimen, with the beak very prominent, and almost vertically truncated at 

 the posterior extremity. 



Fig. 2 b. A cast having the markings almost entirely obliterated, and showing a fold or depres- 

 sion along the hinge-line. 



Fig, 2 c. A specimen partially preserving the shell, which is alated on the posterior dorsal mar- 

 gin, and slightly acute in front. 



Fig, 2 d. A fragment preserving an impression of the shell, showing the concentric and radiating 

 lines. 



Fig, 2 e, A portion of the same enlarged. 



Position and locality. This species occurs at Medina, in a thin stratum charged with uni- 

 valves, about sixty feet from the surface of the rock. It occurs likewise at Lockport, in the same 

 position, or at a less distance below the surface. {State Collection.) 



^*^ I am unable to determine the true relations of this shell. It is allied to Modiolopsis, but its 

 absolute generic relations may be somewhat different. 



