1878.] 19 {Rathbun. 
shell, the lateral margins are straight and sub-parallel posteriorly, and 
anteriorly form, with the anterior margin, quite a regular semi- 
elliptical curve, or the anterior margin may be nearly straight along 
the middle. In other cases the curve of the margins from one cardinal 
angle to the other is quite regular, and subcircular or semi-elliptical. 
Ventral valve generally very convex, and often gibbous or inflat- 
ed; but at times much depressed. In specimens where the length 
and breadth of the shell are nearly equal, the valve is usually very 
regularly convex, and the curvature from side to side, and from the 
beak to the front, are subequal. The broader forms are frequently 
slightly, but broadly, flattened along the middle, along which there is 
often ashallow, undefined median depression of variable breadth, al- 
ways beginning some distance anterior to the beak. At the cardinal 
extremities, and for some distance in front of them along the lateral 
margins, the valve is always flattened. In the larger specimens the 
width of the hinge area at the beak is from about 1 mm. to 1.5 mm. 
at the cardinal angles it narrows down to about one-third its width 
at the beak. The beak is small and only slightly extended be- 
yond the hinge margin. In the interior of the shell, there exists a 
thin, but distinct, median septum, equal to one-fifth or one-half the 
length of the valve, and sometimes quite heavy, but not elevated. 
Spines are indicated on a single specimen only. On this are pre- 
served the bases of five spines, sub-equidistant from one another, and 
inclining outward at an angle of about fifty-five degrees. 
Dorsal valve strongly concave, generally most concave near the 
middle, and having the curve from the beak to the anterior margin 
quite regular, but sometimes more rapid near the front. The cardi- 
nal angles are flattened, and along the middle there is frequently an 
undefined ridge, corresponding with the depression in the ventral 
valve. A few specimens show slight impressions of a medium sized 
cardinal process, but no septum occurs. The beginnings of the vas- 
cular impressions, directly in front of the cardinal process, and two 
short, slightly diverging ridges anterior to, and included between, the 
impressions are often preserved. 
The raised lines of the shell are quite prominent, and slightly 
rounded or sub-angular on top; they are narrower than the interven- 
ing depressions, which are rounded in the bottom. The surface is 
often smooth at the cardinal extremities, and old shells are frequently 
much thickened along the hinge line. Four specimens of different 
shapes and sizes afford the following measurements, in connection 
