MERXSTELLA OE THE HAMILTON GROUP. 
specimens, the beak of M. haskinsi 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 spee'es 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 muscular 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. 
Geological 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. 
Meristella rostrata. 
PLATE L. 
Jllrypa rostrata : IIall, Report on Fourth Geol. District of New-York, 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 
f 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. 
Surface of partially exfoliated valves marked by conspicuous but slen¬ 
der and distant radiating striae : fine and closely arranged concentric 
striae 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. 
