190 PALEONTOLOGY OF NEW- YORK. 



and lateral margins. Hinge line equal to the greatest width of the 

 shell, crenulated. Area linear, vertically striated. Foramen small, tri- 

 angular, closed in full-grown individuals. 

 Surface marked by fine obscure closely arranged radiating striae, crossed 

 on the depressed part of the valves by small regular concentric 

 wrinkles. 



The interior of the ventral valve shows a narrow almost linear area, the inner 

 margin of which is crenulated, the crenulations extending on each side more than 

 half way from the centre to the cardinal extremities. In old shells there is no evi- 

 dence of foramen, but a slight depression in the inner edge of the area (which may 

 be accidental ), with the two cavities beneath for the reception of the processes of 

 the opposite valve. The vascular area is somewhat broadly ovate or cordate, with a 

 prominent imprint of the adductor muscles, from which extends a narrow ridge 

 through the centre of the muscular area. Another individual, apparently identical 

 with this one, shows the muscular area more distinctly divided through the centre. 

 An impression of the interior of the dorsal valve shows the strong imprints of 

 adductor muscles, with cavities made by the teeth or cardinal processes. 



This shell bears a general resemblance to Strophomena depressa. It differs, how- 

 ever, remarkably from that species in its resupinate character, the convexity and 

 concavity of the valves being in the opposite direction : the area of the ventral valve 

 is also wider, and the cardinal margin is crenulated. 



PLATE XXr. 

 Fig. 5 a. Tlie dorsal valve, showing obscurely the concentric wrinkles and the punctate 



surface where the shell is exfoliated. 

 Fig. 6 a. Interior of the ventral valve. 

 Fig. 6 b. Profile of the preceding. 



Fig. 6 c. Enlargement of the crenulations of .the hinge line of 6 a. 

 Fig. 7 a. Interior of a ventral valve. 

 Fig. 7 b. Profile of the same. 



PLATE XXin. 

 Fig. 1 a, b, c. Ventral, dorsal and profile views of an entire specimen. 

 Fig. 2 a, b, c. Dorsal and profile views of a full-grown individual. 

 Fig. 2 e. Enlargement of the fine equal striae. 

 Fig. 3."^ Impression or cast of the interior of the dorsal valve. 



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

 group : Helderberg mountains. 



