m 
PALAEONTOLOGY OF NEW-YORK. 
ly incurved, the beak gently incurved over the area (and, in some 
specimens, more incurved than is shown in the figures) ; the beaks of 
the two valves are approximating. 
The surface is marked by fine equal or subequal striae, and a few 
scarcely lamellose concentric lines of growth, the bifurcations of the 
striae producing scarcely visible inequalities. The texture of the shell is 
punctate. The number of striae in the space of two lines varies from 
twelve to sixteen. 
The cast of the dorsal valve shows a broadly ovate muscular impression, 
which is slightly divided by a longitudinal depression : from the lower 
part of this muscular impression the vascular markings extend in a 
narrow band, slightly spreading and diverging below. In this respect it 
differs from the 0. propinqua and 0. multistriata, where the muscular im¬ 
pression is usually more quadrate, and the vascular impressions are al¬ 
ways diverging from the muscular area and are more divided. In some of 
the younger specimens of O. propinqua , the muscular impression of the 
dorsal valve is more ovate than that given fig. 39, Plate v. 
The muscular impression in the ventral, valve of this species is narrow 
ovate-lanceolate, with the limits strongly defined. 
Externally this species differs from 0. propinqua in its finer and more even 
strise, as well as in its external form; the greatest width in that species being 
below the middle, while the margin thence, towards the hinge-extremities, is 
more direct. In 0. tulliensis, the greatest width is about the middle, and the 
outline is pretty regularly curved. 
The figures on Plate vii illustrate the usual form and character of adult speci¬ 
mens. The younger ones are less gibbous, but they are usually in such imperfect 
condition that my collections have not afforded specimens fit for illustration. 
Geological formation and locality. In the Tully limestone at Tully and Tinker’s 
falls, Onondaga oounty, and at Ovid in Seneca county. 
