FOSSILS OF THE NIAGAEA GROUP. 133 



only. The features of the muscular imprints and the cavities left by 

 the removal of the septa and other processes are well marked. The form 

 IS triangularly ovate, or sub-quadrangular. Valves convex, nearly 

 equally so; the front of the ventral being bent upwards, forming the 

 sinus of this valve, and*causing a slight elevation or fold on the dorsal, 

 which extends about half the distance to the beak. Cardinal margins 

 of the ventral valve angular, corresponding to the angulation of this 

 part of the shell, while the surface of the valve across the umbo is 

 nearly flat from side to side, except for the projection of the muscular 

 impression. The beak is short and not recurved, consisting only of the 

 filling of the rostral cavity. Dorsal valve most convex on the umbo and 

 along the middle of the valve, and in old specimens rapidly sloping to 

 the lateral margins, the beak showing a strong slit corresponding to the 

 median septum. 



Formation and locality : In limestones of the Niagara group, Springfield, Ohio. Col- 

 lection of Prof. E. Orton. 



Genus ATRYPA, Dalman. 

 Atrypa nodostriata. 



Plate 7, flgs. 12-14. 

 Atrypa nodostriata, Hall; Pal. N. Y., Vol. II., p. 272, pi. 54, fig. 2, 1852. 



Shell small; transversely very broad oval in outline; bi-convex in 

 profile, somewhat unequally so, with a low rounded elevation on one 

 valve and a corresponding depression on the other. Ventral valve less 

 convex than the opposite ; beak small and projecting ; center of the shell 

 marked by a rather wide, very distinct mesial depression, extending 

 from near the beak to the front of the valve, which is abruptly bent up- 

 wards, forming a broad, rounded, linguiform extension, fitting into the 

 elevated portion of the other valve; sides of the mesial depression sub- 

 angular; outside of which the surface of the valve is somewhat de- 

 pressed or concave to the sides and cardinal margins. Dorsal valve 

 rather regularly convex, the middle portion elevated, forming the fold, 

 which is not conspicuous beyond the center of the valve, but is some- 

 what distinctly marked and rapidly narrowed. 



Surface marked with bifurcating, rugose, radiating striae, which often 

 form fasciculfe at or about the middle of the shell. The striae are strong 

 and apparently rounded, most distinct oti the dorsal valve, and some- 

 what strongly marked toward the front of both valves. 



