SB [Senate 



valve shorter than the ventral j beak incurved. Surface marked 

 by twelve to sixteen somewhat angular plications on each valve, 

 two or three of which are slightly depressed on the middle of 

 both valves, so as to produce sometimes a faint emargination m 

 front; the depressed plications smaller than the others, and 

 often becoming obsolete before reaching the beak. 



The globose form and slightly elevated plications of this shell, as well 

 as its more distinctly imbricating lamellse, will serve to distinguish it from 

 another species in the same rock. 



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

 berg group, Albany county. 



WaLDHEIMIA FORMOSA, 



Pal. N.Y. Vol. iii, pi. 36, f. 2. 



Shell longitudinally ovate : ventral valve tapering towards the 

 beak ; beak prominent, rounded, arched or incurved, truncated 

 at the apex by a round perforation, one side of which is formed 

 by the deltidium : dorsal valve convex, sometimes most pro- 

 minent near the umbo ; beak incurved. Surface marked by 

 eighteen to twenty-two or twenty-three simple angular or some- 

 what rounded 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. 

 Shell marked by fine imbricating concentric lines of growth. 



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 obsolete or nearly disappear before reaching the beak. 



There are rarely individuals, as fig. 2 of pi. 36, which approach more 

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

 rotund in form, and the beak larger. 



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

 berg group, Albany county. 



