Se 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 striaj, and a few 

 scarcely lamcUose concentric lines of growth, the bifurcations of the 

 stria) 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 

 striae, 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 county, and at Ovid in Seneca county. 



