MERISTELLA OF THE HAMILTON GROUP. 307 



specimens, the beak of M. hasHnst is sufficiently elevated to allow the deltidial 

 covering of the fissure to be visible. In specimens where the surface is well pre- 

 served, the striae offer a distinguishing character. 



The figures 23-35, Plate xlix, illustrate this species in its principal varities of form and 

 size. In figures 32 & 34, the deltidial plates beneath the foramen are visible. 



Figure 35 is a partial cast of the dorsal valve, showing the filling of the rostral cavity 

 and the form of the mnscnlar impression. 



Figure 36 is a partial cast of the dorsal valve, preserving in part the muscular impression. 



The fossil figured in the Report of the Fourth District, p. 202, fig. 5, is probably an exfo- 

 liated specimen of this species. 



GeoJogical formation and localities. This species occurs in the shales of the 

 Hamilton group, on the shores of Seneca lake^ at Geneseo, York, Moscow and 

 Leroy, in Livingston county, and at Eighteen-mile creek in Erie county. 



meiistella rostrata. 



PLATE L. 



•Alnjpa rotlrata : IlALt, Report on Fourth Geol. District of New-Yorlt, p. 202. f. 2. 1843. 



Shell small or of medium size, ovate, rounded below and often subat- 

 tenuate above ; valves subequally convex ; length and breadth about 

 as eleven to eight or nine. 



Ventral valve scarcely more convex than the dorsal, a little gibbous 



above the middle, gradually curving to the basal and baso-lateral 



margins, while above the middle the edges are abruptly incurved : 



umbo prominent; beak more or less strongly incurved over the umbo 



 of the opposite valve ; the lower part of the valve, and sometimes 



r the entire length, marked by a shallow rounded sinus. 



Dorsal valve regularly convex, somewhat more prominent above the 

 middle ; slightly or not at all elevated in front from the influence of 

 the sinus of the ventral valve. 



Surf .ICE of partially exfoliated valves marked by conspicuous but slen- 

 der and distant radiating striaa : fine and closely arranged concentric 

 strife mark the entire surface, and towards the margins these are 

 crowded in strong folds. 

 Shells of this species are scarcely three-fourths of an inch in length 



in the largest specimens observed. The proportions of length and breadth 



vary considerably. -- 



