468 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ing a completely divided hinge-plate (cardinal process) in the 
brachial valve, lightly impressed muscle-scars, and only a rudi- 
ment of a dorsal median septum. The shell is short, with a wide 
hinge, as in the young stages of Rensselaeria. 
Type, Prorensselaeria nylanderi, sp. nov. 
Prorensselaeria nylanderi, sp. nov. 
PI. 6, figs. 3, 4. 
Type, no. 15076, collection Boston Society of Natural History. 
Description. — Shell approximately circular in outline, biconvex, 
with numerous, subequal fine plications. Valves subequal, the 
pedicle somewhat the deeper, the greatest convexity of both being 
a short distance in front of the umbones; the shells flatten for- 
ward so that they approach each other at a small angle and the 
front is thin. The width at the hinge (on the cast) is about three- 
fourths the greatest width, which is a little behind the middle of 
the brachial valve. The plications are rounded, coarser in the 
middle of the front than on the lateral slopes, are separated by 
furrows of less than their own width, and become faint, or entirely 
obliterated, on the highest parts of the valves. There are a few 
strongly marked growth lines, but no concentric striae. 
The interior of the brachial valve, as reproduced in wax from 
the natural cast, shows a large, double cardinal process, the parts 
separated by a groove which reaches to the floor of the valve. 
Each moiety of the process appears as half a cone, the upper an- 
terior margin bearing the brachial support. These cones are not 
in contact and their surfaces are smooth, so that there is no chance 
that they have been separated by the loss of a connecting plate. 
The visceral groove expands and becomes deeper in front of the 
cardinal process, and is bordered by a pair of low ridges which 
die out abruptly just before reaching the bottom of the valve. 
Behind these ridges and the visceral furrow are the narrow, elon- 
gate, faintly impressed scars of the adductor muscles, bounded 
laterally by sinuous, linear, scarcely elevated ridges, and separated 
by a sharp, almost indistinguishable, rudimentary median septum. 
The pedicle valve is slightly distorted, the posterior end having 
been pushed a little toward the front, causing a slight buckling, 
shown by a curving wrinkle across the most convex part of the 
shell. This has brought the muscular area into a position almost 
