LOWER HELDERBERG ROCKS. 853 



Fig. 2 a, b. Dorsal and profile views of a young specimen, the beak of which is straight. 



Fig. 2 c. Dorsal view of a large specimen, where the beak is scarcely incurved. 



Fig. 2 rf, e,f, g. Specimens presenting the ordinary characters of the species. 



Fig. 2 h. i. Dorsal and profile views of a more elongated form, which is slightly wider to- 

 wards the upper part of the shell. 



Fig. 2 k, I, m. Dorsal, profile and front views of a very gibbous specimen. ' 



Fig. 2 n, 0, p. Views of specimens which are much broader towards the upper part of the 

 shell, very gibbous, and having the beak closely incurved. 



Geological position and locality. In the higher part of the Shaly limestone, and 

 more compact beds just beneath the Upper Pentamerus limestone of the Lower 

 Helderberg group : Albany and Columbia counties. 



Rensselseiia seqairadiata. 



Plate XLV. Fig. 3 a - g. 

 JUrifpa ctquiradiata ; Conrad, Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, pa. 266, pi. 16, f. 17. 



Shell elliptical or subovoid : valves nearly equal j without mesial fold 

 or sinus. Ventral valve usually the more convex, gibbous, and often 

 subangular along the centre of the upper part of the shell : beak much 

 extended above the opposite, and incurved. Dorsal valve usually less 

 convex than the opposite, and sometimes depressed convex : beak in- 

 curved beneath that of the opposite valve. 



Surface marked by simple regular radiating striae, which are much more 

 conspicuous towards the margin of the shell. 



The R. mutabilis resembles this species in its form and striae; but among a large 

 number of specimens examined, none have been observed larger than those figured. 



Fig. 3 a, b, c. Dorsal, ventral and profile views of a larger specimen, which is proportionally 

 longer than the prevailing forms. 



Fig. 3 d. Front view of the same. 



Fig. 3 «, /, g. Dorsal, ventral and front views of a larger specimen, which has a proportio- 

 nally greater breadth than the preceding, resembling in this respect the figure of 

 Mr. CoNKAD cited above. 



Geological position and locality. In the Upper Pentamerus limestone of the Lower 

 Helderberg group : Schoharie, Carlisle, Cherryvalley. 



