334 
PALAEONTOLOGY OF NEW-YORK. 
Fig. 1 in. Interior of the dorsal valve. 
Fig. 1 n. Interior of the ventral valve, showing the teeth, muscular area, and the vascular 
impressions. 
Fig. 1 o. The ventral valve of a smaller individual. 
Fig. 1 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 Riyssfjeria. 
For description and illustration of this genus, see the same under Oriskany 
sandstone. 
Rensseljeria miitabilis (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. Yentral 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. 
Surface 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 striae 
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 beak of the 
ventral valve is generally closely incurved. 
