ORISKANY SANDSTONE. 439 



Rliynchonella oblata. 



Plate CII. Fig. 1 ? & 2 a - d. 

 RhynchoneUa oblata : Hall, Regents' Report of 1856, p. 86; Palaeozoic Fossils, 1857, p. 46. 



Shell subcircular, somewhat compressed. Dorsal valve the larger, de- 

 pressed convex, declining with a gentle curve towards the lateral 

 margins, rising in front into a broad round undefined mesial fold : beak 

 somewhat incurved. Ventral valve much compressed, depressed convex 

 in the middle and umbonial regions, and depressed into a broad shallow 

 undefined sinus in front. 



SuEFACE marked by seventy-five to eighty coarse strias, which occasionally 

 bifurcate, and of which twelve to fourteen or more mark the mesial 

 fold and sinus. 



The specimen fig. 1, which is referred with doubt to this species, preserves a part 

 of the shell : the other specimens are casts. The muscular area of the ventral valve 

 is comparatively small, and the space for the adductors quite narrow. The ramifying 

 vascular impressions are beautifully preserved in a single specimen, radiating from 

 the muscular area and extending to the cardinal line, where they are strongly 

 marked. In the dorsal valve the medio-longitudinal septum extends from the beak 

 nearly halfway to the base of the shell, the cast sometimes showing the marks of 

 muscular impressions. The impressions of the radiating striae are preserved on the 

 lower half of the casts. 



Fig. 1. A young individual, retaining a part of the shell on the dorsal valve. 



Fig. 2 a. Front view, showing the broad shallow sinus. 



Fig. 2 6. The ventral side of another specimen, showing the muscular area and the ramifying 

 vascular impressions. ( The latter are represented in lines too sharply defined : 

 the two sides are not connected by a sharp line, as shown in the figure ; and 

 their extension towards the front of the shell is in shallow depressions, which 

 gradually become obsolete.) 



Fig. 2 c & (/. Dorsal and cardinal views of the same specimen. 



Geological position and locality. In the Oriskany sandstone : Albany and Scho- 

 harie counties. 



