228 
PALAEONTOLOGY OF NEW-YOBK. 
follow; but I have not been able to find reliable characters for separation, among 
a large number of specimens. 
This species resembles R. mutabilis in form, but may be distinguished by its more 
angular and more strongly elevated plications, of which there are fewer on the 
mesial sinus and elevation. 
Fig. 1 a, b, c. A young individual and one of medium size, having smaller plications than 
usual, and a larger number upon the mesial sinus. 
Fig. 1 d, c, f. Larger individuals, having stronger plications than the preceding, and a 
larger number upon the mesial fold and sinus than those which follow. 
Fig. 2 a - c- Figures presenting the ordinary character of the smaller individuals of this 
species. 
Fig. 2 f - t. Illustrations of the gradations in size, form, and proportions of this species. 
Fig. 2 w, x, y. A cast of this species, showing the form of the muscular impression in the 
ventral valve, the median plate of the dorsal plate, and the alveolar cavities. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Schoharie, Carlisle and other places. 
Plate XXXI. Fig. 3. 
Shell transversely oval, subpentagonal. Ventral valve depressed convex, 
very abruptly deflected towards the opposite valve on the lateral 
margins : beak small, depressed on the outside and subangular along 
its lateral slopes, closely incurved over that of the opposite valve. 
Dorsal valve much the larger, very prominent in front, obliquely 
declining towards the beak : beak depressed, incurved. 
Surface ornamented by from twenty-five to thirty-three simple subangu¬ 
lar plications, seven or eight of which are elevated towards the front 
of the dorsal valve into a somewhat distinct mesial prominence, and 
from six to eight of those on the middle of the ventral valve are de¬ 
pressed so as to form a broad undefined sinus, which scarcely extends 
beyond the centre of the valve towards the beak, but is prolonged in 
front, and abruptly bent upwards nearly at right angles to the dorsal 
valve into a distinct linguiform extension. The plications are marked 
in front by the usual longitudinal depressed line along the centre of 
each, and extremely fine regular zigzag lines corresponding to the 
