256 
PALEONTOLOGY OF NEW YORK. 
tion is usually lost and only a median division by a slight longitudinal septum is 
discernible. The entire area does not extend more than one-third the whole 
length of the valve. Over the post-lateral slopes are numerous fine, irregularly 
ramifying genital sinuses. 
The brachial valve is considerably the less convex and is often flattened. 
The beak is minute and usually obscured by the overlapping pedicle-valve. 
The hinge-plate is large, flat, triangular, sometimes thin, often thickened on its 
posterior portion and resting on the bottom of the valve. It is separated from 
the lateral shell-walls by narrow dental grooves widening at their extremities. 
Normally this plate appears to have been perforated by a visceral foramen 
entering at the underside and opening at or beneath the apex of the beak. 
This perforation is however frequently filled by adventitious deposits though 
traces of it are discernible in the oldest shells, and in casts of the interior the 
filling of the tube is often preserved. The median portion of the plate, lying 
between two vertical supporting lamellae resting on the bottom of the valve, is 
preserved in the earliest and simpler species, but in the larger and later forms 
of the genus, is frequently resorbed, giving the plate the appearance of being 
composed of subtriangular, discrete lateral halves. 
The crura are the continuation of the upper portions of the supporting septa. 
Neither these nor the rest of the brachidium have been seen in the type species, 
R. ovoides, of the Oriskany sandstone,"^ but in R. Marijlandica, the crura are 
broadened just beyond their base of attachment, and from their upper angles 
are given off the jugal processes which are long, pointing upward and inward, 
but not uniting. From the lower angles the descending arms take their origin, 
following the curves of the valve, diverging for a short distance, thence abruptly 
approaching, and uniting to form a broad elongate, acutely triangular plate, 
which is not supported by a median septum, or otherwise connected with the 
valve. From the middle of the posterior margin of the plate arises a small 
rod-like process, which extends for a short distance upward toward the crura. 
The entire length of the brachidium is nearly two-thirds that of the valve. 
* The figui es of ihe brachial supports ascribed to It. ovoides in the Twelfth Report on the Slate Cabinet 
of Natural History, p. 41, represent Ihe species iJ. Marylandica, which at that date had not been separated 
from R. ovoides. 
