LOWER HELDERBERG ROCKS. 
3®f> 
sharp interlocking edges of the front and lateral margins of the valves. 
These fine striae doubtless represent lines of growth, which have be¬ 
come wholly obsolete on other parts of the shell. 
In this species the two plications bounding the sinus of the ventral valve, and 
the mesial elevation of the dorsal, sometimes bifurcate towards the beak or middle 
of the valves, one becoming obsolete on the front. It resembles some of the Bohemian 
forms which are considered varieties of R. wilsoni , but is a larger and relatively 
broader shell, the mesial elevation is more distinct, and the general form is less 
rotund. It is possible that more extensive collections may prove this species to be 
an extreme variety of the preceding. 
Fig. 3 a , b. Dorsal and veDtral views of a well-marked individual of this species. 
Fig. 3 c, cl. Front and cardinal views of another individual. 
Geological position and locality. Shaly limestone of the Lower Ilelderberg group, 
Albany and Schoharie counties. 
IlBiyiiciimiella pyramidata (n. s.). 
Plate XXXII. Fig. 1 & 2. 
Shell pyramidal, subpentagonal; outline subquadrangular. Yentral valve 
nearly flat or depressed convex, more or less abruptly deflected at the 
margins towards the other valve : beak prominent, flattened, and 
closely incurved over that of the opposite valve. Dorsal valve very 
gibbous, declining from near the front towards the beak : beak angular, 
incurved, flattened; lateral slopes with a distinctly impressed suboval 
space beneath the beaks, which is bounded by an angular fold extending 
from the beak downwards to the margin of the valve at the first strong 
plication. 
Surface marked by from thirteen to twenty-two simple strong subangular 
plications, four to six of which are more or less elevated towards the 
front of the dorsal valve into a mesial prominence, and three to five 
depressed on the ventral valve so as to form a shallow sinus, and 
produced in front into a distinct linguiform projection. 
The plications on the front of this shell have but very faint traces of the longi¬ 
tudinal depressed lines so common in this type of Rhyn chon ellce, though occasionally 
