916 PAL.EONTOLOGY OF NEW-YORK. 



Surface marked by eighteen to twenty-two or twenty-three simple 

 rounded or rarely subangular plications, two or three of which are 

 much smaller and slightly depressed on the middle of each valve, so 

 as to form a faint narrow sinus extending nearly or quite to the apex 

 of the beaks, and giving a slight emarginate outline to the front. 

 Surface marked by fine imbricating concentric lines of growth, which 

 become strong lamellae towards the margins of the shell : shell granu- 

 lose. 



This species differs from the preceding in its more elongate form, larger size of 

 full-grown individuals, and less rugose imbricating lines of growth. The plications 

 are less strongly developed and more numerous, and the central ones become obso- 

 lete or nearly disappear before reaching the beak. 



There are rarely individuals, as fig. 2/ of Plate xxxvi, which approach more 

 nearly in character to the preceding species; but they are always less rotund in 

 form, and the beak larger. 

 Fig. 2 a- e. Young shells of this species, the plications fine and equal. 



The foramen is represented as extending below the beak, and having a tri- 

 angular form from the absence of the deltidium : this, however, is an accidental 

 condition, as the young shells frequently preserve the deltidium entire, and 

 present the round perforation as in the beals of old shells. 



Fig. 2 f. An individual having strong angular striae more like the preceding species, but 

 with the elongate form and narrow beak characteristic of this one. 



Fig. 2 g - k. Dorsal, profile, and front views of specimens of the ordinary size. 



Fig. 2 I - t. Individuals of larger size and very symmetrical form. 



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



group : Helderberg mountains, Albany county. 



Trematospira dewejl (n.s.). 



Plate XXXVI. Fio. 3 a - A. 

 Waldheimia deweyi : Descr. of New Species of Pal. Fossils in Regents' Report for 1850, p. 49. 



Shell depressed subglobose, sometimes subquadrilateral with the sides 

 curving : valves nearly equal. Ventral valve a little the most promi- 

 nent towards the umbo, having a narrow faint sinus from near the 

 beak to the front, where it sometimes produces a slight sinuosity : 

 beak perforate, extending a little above the opposite beak, upon which 

 it is closely incurved. Dorsal valve symmetrically arched, without 

 mesial elevation. 



