RHYNCHONELLIDiE OF THE HAMILTON GROUP. 
339 
and gently defined below the middle of the length of the shell. The 
umbo is prominent, the beak small and neatly incurved over the umbo 
of the opposite valve. 
Dorsal valve much more convex than the ventral, the greatest gibbosity 
a little above the middle, gently curving to the sides and baso-lateral 
margins; the centre elevated in a broad mesial fold. 
Surface marked by numerous rounded or subangular plications, of which 
from three to six or seven are depressed in the sinus and a correspon¬ 
ding number elevated on the fold. The plications of the fold and sinus 
are always bifurcating, those of the sides simple or obscurely bifur¬ 
cating, and all usually becoming obsolete at one-third to one-fourth the 
length of the shell from the apex; concentrically marked by raised 
thread-like strise, which sometimes become squamose imbricating 
folds or lines of growth. 
The cast of the ventral valve shows a narrow rostral cavity and slen¬ 
der dental plates, with a lanceolate or lance-ovate muscular area. The 
ovarian spaces are papillose-striate, and outside of these are the rami¬ 
fying vascular imprints. The cast of the dorsal valve shows a median 
septum reaching more than half the length of the shell, divided above, 
leaving a triangular pit. The muscular imprint is narrow, elongate, and 
marks the surface on each side of the septum more than half-way to the 
base. 
This species presents considerable variety in its surface characters, varying 
from specimens with few perceptible plications on the sides of the shell, to those 
with six. or seven on each side, and from three to seven in the mesial sinus. In 
some of the specimens in the Hamilton shales, the plications are numerous, suban¬ 
gular, reaching to the apex of the shell, and showing distinct bifurcations in 
those of the lateral as well as the mesial portion of the valve. 
In order to make a comparison with the Canadian species, I have procured 
specimens from Bosanquet and Widder in Canada West; and I find the same 
variations among them as in those of New-York. In seventeen specimens examined, 
there were two corresponding essentially with the figure of Mr. Billings (loc. 
cit.), except that the plications are obsolete towards the beak : one of these 
specimens is about an inch in length and breadth, and the other a little less. In 
other specimens of nearly the same size, there are four or five plications on the 
