PALEONTOLOGY OF NEW-YORK. 
remains of very fine closely arranged zigzag stria- are seen crossing them near the 
margins of the valves. 
In some of its varieties, this species approaches the R. nucleolata ( Plate xxxi, 
fig. 1 & 2), but differs conspicuously in being usually larger, and in its more an¬ 
gular outline and much stronger plications. The beaks are also more prominent in 
this species, and the dorsal valve is more extremely elevated near the front : there 
are also differences in the visceral impressions. These differences of character are 
constant through a very extensive series from the young to the old shells. 
This species belongs to the type of R. wilsoni , and may be said to bo one step 
farther removed from that species than R. nucleolata ■ or, in other words, bearing 
about the same relation to the latter species which that one does to R. wilsoni. 
Fig. 1 a - cl. Young individuals of this species. 
Fig. 1 e - z. Illustrations of the varying forms of this species, from those of medium size 
to the largest and most gibbous forms that have been observed. 
Fig. 2 «. A cast of the ventral valve of this species, showing the muscular impression 
smaller than in R. nucleolata , Plate xxxi, fig. 2 x. 
Fig. 2 6. A cast of the dorsal valve of the same species. 
Fig. 2 c. Interior of the ventral valve, which partially preserves the form of the muscular 
imprint. 
Geological position and locality. Shaly limestone of the Lower Helderberg group, 
Albany county. 
Plate XXXIII. Fig. 1 a-p. 
Shell varying from transversely oval to sub triangular. Ventral valve 
depressed convex : beak somewhat prominent, depressed, closely in¬ 
curved over the opposite. Dorsal valve more gibbous : beak incurved, 
not prominent. 
Surface marked by twenty-four to thirty-six plications, six or eight of 
which are elevated in front of the dorsal valve so as to form a rather 
distinct mesial prominence, rarely extending beyond the middle of the 
valve. On the ventral valve, five to seven of the plications are de¬ 
pressed towards the front into a more or less distinctly defined sinus, 
and prolonged, forming a mesial projection which is elevated in the 
front of the other valve. Near the junction of the valves in front, very 
fine closely arranged lines of growth are visible. 
