ORISKANY SANDSTONE. 
429 
Cyrtia rostrata. 
Plate XCYI. Fig. 1 - 6; and Plate XCYIII. Fig. 8 a, b. 
Cyrtia rostrata : Hall, Regents’ Report for 1856, p. 64; Palseozoic Fossils, 1857, p. 24. 
Shell triangularly pyramidal, more or less elevated : hinge-line straight, 
equalling or greater than the greatest width of the shell below, and 
sometimes extending into acute angles. Ventral valve much elevated 
at the beak, a distinct sinus extending from beak to front : beak simple, 
angular, not incurved. Dorsal valve convex, semicircular; mesial fold 
moderately elevated, rounded or slightly flattened, and marked with a 
faint longitudinal depressed line : beak scarcely elevated above the 
cardinal margin : area broad triangular, plane, or rarely subarcuate ; 
foramen narrow, extending to the apex of the beak of the ventral valve, 
partly closed above by a central plate. 
Surface marked by five to eight or nine elevated subangular costae on 
each side of the middle, concentrically marked by imbricating lamellae 
and finer striae, which are crossed by fine radiating striae; the entire 
surface, in perfect specimens, being papillose. 
The interior of the dorsal valve shows two strong crural processes with 
the dental fossets, and a cardinal process which is more or less developed. 
In the ventral valve, the dental lamellae, uniting at their bases, are con¬ 
tinued in a median septum, which is extended nearly to the anterior 
margin of the shell : the same septum, extending into the triangular 
cavity of the foramen, approaches to the plane of the area, which, in well- 
preserved specimens, is distinctly striated vertically. A single specimen 
preserves the internal spires. 
This species bears some resemblance to Cyrtia ( Spirifer ) heteroclita; but it has 
a greater number of plications, and is a larger and coarser shell. 
The youngest specimen observed of this species has six plications on each side of 
the mesial fold, while the older ones have eight or nine : the plications vary from 
angular to round. 
