No. 109.] 107 



truncate or rounded extension. Surface smoothj or marked by 

 faint concentric lines of growth. 



Young specimens of this species miglit be mistaken for the young of P. 

 galeatus, which arc, like this, destitute of plications. Its form, ho\Ycver, is 

 much more elongated, and the beak more extended than in P. gahatus. 



Geological posilio?i and locality. Upper Pentamerus limestone of the 

 Lower Helderberg group, being the fossil which characterizes the higher 

 compact beds of the group, as P. gahatus does those of the lower beds of 

 the same. 



Leptoccelia CONCAVA. 

 Pal. N.Y. Vol. iii, pi. 38, f. 1 to 7. 



Shell ovate or suborbicular : ventral valve convex, elevated along 

 the middle into a mesial prominence, which extends to the 

 umbo; beak small, incurved beyond the hinge-line : dorsal 

 valve near the lateral margins, depressed in the middle, forming 

 a shallow undefined sinus which is deeper in the centre than at 

 the front, and rapidly diminishes towards the umbo ; beak 

 straight : area small ; foramen triangular and extending to the 

 apex of the beak, sometimes closed below by a deltidium. Sur- 

 face marked by fourteen to seventeen strise, which sometimes 

 bifurcate : the one on the middle of the mesial fold of the ven- 

 tral valve is generally smaller than the others, giving a slightly 

 grooved appearance along its centre quite up to the beak. 



The sinus in the dorsal valve of this shell widens so rapidly from the 

 '^eak towards the front, and is so much deeper in the centre than near the 

 6eak and at the front of the shell, that it gives in some instances a marked 

 concavity to this valve. 



This species may be considered a representative of Tcrehratnla duboisi 

 of De Verneuil ( Geologic de la Russie, pi. x, f. 16 ) ; from which it 

 differs in its more rotund outline, and in the greater concavity of the dorsal 

 Talve. 



Geological position and locality. Shaly limestone of the Lower Helder- 

 berg group, Albany county. 



