LOWER HELDERBERG ROCKS. 169 



Fig. 1 a, b, c. Views of small individuals of this species, from Maryland. 



Fig. 2 a, h, c, & 3. Larger individuals, from New- York, showing the ventral and dorsal 



sides, profile and area. 

 Fig. 4. The interior of the dorsal valve of the smaller individuals. 

 Fig. 5. Interior of the ventral valve. 

 Fig. 6 a, h. Dorsal and ventral valves of a cast of the same species. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Albany county; and in the same position, and also 

 in the Oriskany sandstone, Cumberland, Maryland. 



Orthis sabcarinata (n. s.). 



Plate XII. Fig. 7 - 21. 



Shell somewhat transversely oval, often nearly circular or quadrangular, 

 plano-convex. Dorsal valve more or less flattened, with a distinct 

 depression along the middle, which becomes wider towards the base, 

 producing a sinus in the margin of the shell : beak scarcely rising 

 above the hinge line. Ventral valve very convex, strongly elevated or 

 subcarinate along the middle : beak small, incurved. Area narrow, 

 linear, one-half to two-thirds the width of the shell. The depressed 

 line along the middle of the dorsal valve, and the ridge upon the ven- 

 tral valve, very frequently diverge from the central line, though the 

 specimens do not appear to have suffered any distortion from pressure. 



Surface finely striated : striae curving upwards towards the lateral and 

 cardinal margins, equal or alternating in size, and not unfrequently 

 fasciculate ; concentrically marked by strong lines of growth towards 

 the margin, and, in well-preserved specimens, by fine striae over the 

 entire surface. 



Interior of the dorsal valve with an obscurely double impression for 

 the adductor muscles on each side of a low sharp elevation, with sides 

 subparallel : cardinal process strong and bifid at the extremity, with 

 each division bilobed ; brachial processes strong, divergent ; vascular 

 impressions preserved in the cast. The ventral valve shows, beneath 

 the beak, a strong triangular cavity, with an abrupt groove at each 



[ PALiEONTOLOOT III.] 22 



