No. 109.] 



43 



which is sharply incurved over the area ; area narrow, length 

 about half the width of the shell. Surface striated ; striai some- 

 what tubular, prominent, fasciculate, increasing by implantation 

 and bifurcation, extremely curved towards tlie margins, and 

 presenting at somewhat regular intervals small tubular pore-like 

 openings upon the surface ; radiating striae, when not worn, 

 crossed by prominent concentric striae, and, rarely, by stronger 

 lines of growth. 



In the usual condition of the specimens, the concentric striae are only 

 visible in the depressions between the radiating striae, and sometimes are 

 imperceptible. 



Locality. Helderberg mountains. 



Orthis subcarinata ( n, s.)- 



Pal. N.Y. Vol. iii, pi. 12. 



Shell somewhat transversely oval, plano-convex : ventral valve 

 very convex, strongly elevated or subcarinate along the middle ; 

 beak small, incurved : dorsal valve more or less flattened, witli 

 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 ; 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 ventral valve, very frequently diverge from the central line, 

 though the specimens do not appear to have suffered any distortion from 

 pressure. 



Oethis subcarinata. 



