1878.] 19 . [Rathbun. 



shell, the lateral marpcins 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 broailly, flattened along the middle, along which there is 

 often a shallow, 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 tliis 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 

 hort, 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 subhangular 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 



