aat palj:ontology of new-iork. 



Fig. 1 M. Interior of the dorsal valve. 



Fig. 1 n. Interior of the ventral valve, showing the teeth, moseular area, and the vascular 

 impressions. 



Fig. 1 o. The ventral valve of a smaller individual. 



Fig. I p. A specimen with the dorsal valve partially broken away, showing the internal 

 spires, which, by mistake of the lithographer, are represented as one. 



Fig. 1 r. Front view, showing the arrangement of the spires. 



Geological position and locality. In the Lower Pentamerus limestone, in the Shaly 

 limestone very abundant, and in the Upper Pentamerus limestone of the Lower 

 Helderberg group : Helderberg mountains, Schoharie, Carlisle, Hudson, Catskill, 

 etc.; Pennsylvania; Virginia; Maryland. 



Genus REBi8SEL;£RI.4. 



For description and illustration of this genus, see the same under Oriskany 



sandstone. 



Rcnsselseria nintabilis ( n. s.). 



Plate XLV. Fig. 2 a- p. 



Shell ovate varying to elliptic and obovate, not sinuate on either valve : 

 old specimens sometimes very gibbous, but generally compressed to- 

 wards the anterior border in young individuals : valves nearly equally 

 convex. Veatral valve most elevated near the middle and towards the 

 umbo : beak pointed, subangular along the lateral slopes, arched or 

 closely incurved ; foramen narrow, and extending nearly or quite to 

 the apex of the beak. Dorsal valve slightly less elevated and a little 

 shorter than the opposite : beak not projecting, incurved. 

 SoRFACE marked by twelve to twenty-eight coarse obscure radiating striae, 

 crossed by fine indistinct lines of growth, and sometimes near the 

 border by a few strong concentric undulations : the radiating stria9 

 are usually obsolete on the upper half of the shell. 



This species varies considerably in form, as well as in other characters. Young 

 individuals are generally more compressed near the front, and the beak is more 

 nearly straight ; while older specimens are often quite gibbous, and sometimes 

 marked by very strong concentric undulations : in the latter case, the l)eak of the 

 ventral valve is generally closely incurved. 



