LOWER HELDERBERG ROCKS. 
233 
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 d, 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, /, 7ii. Dorsal, profile and front views of a very gibbous specimen. 
Fig. 2 n, o, 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. 
liensselfleria wquiradiata. 
Plate XLV. Fig. 3 a - g. 
Atrypa (zquiradiata : Conrad, Jour. Acad. Nat. Sci. Philadelphia, Vol. viii, pa. 266, pi. 16, f. 17. 
Shell elliptical or subovoid : valves nearly equal; 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. 
Sueface marked by simple regular radiating strise, which are much more 
conspicuous towards the margin of the shell. 
The R. mutabilis resembles this species in its form and strise; 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 e,f, 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. Conrad cited above. 
Geological position and locality. In the Upper Pentamerus limestone of the Lower 
Helderberg group : Schoharie, Carlisle, Cherryvalley. 
