656 F. C. GREENE 



ing the hinge-line. Immediately under the beak, this valve is at 

 first convex, but becomes concave farther forward. 



Ventral valve. — The convexity of this valve exceeds the concavity 

 of the dorsal valve to a very great extent in some specimens. In other 

 specimens this valve follows the dorsal valve closely. The striae of 

 this valve are similar to those of the dorsal. Shell punctate in the 

 furrows between the striae. What appear to be large punctae are 

 scattered over the surface of this valve. These are lighter in color 

 than the rest of the surface and are largest at the anterior margin, 

 growing smaller and more thickly set toward the beak. They 

 possibly correspond to the spines of Producti. 



The beak projects slightly over the hinge-line. Cardinal area 

 long and narrow; longer than the greatest width of the shell farther 

 forward. It lies at an angle of 45° to the plane of the shell (Plate II, 

 Fig. 12, 13) grooved with horizontal striae; the greater part of the 

 area is on the ventral valve. The hinge areas of the two valves form 

 an obtuse angle. 



Two plates of the cardinal process of the opposite valve partially 

 close the delthyrium. The hinge area of this valve is bordered by a 

 row of seven to eleven spines on each side of the beak, growing larger 

 toward the cardinal extremities. This valve is the larger of the two 

 (Plate IV, Fig. 4). 



Interior of dorsal valve {Plate II, rows j, 4). — Convex. Hinge- 

 line straight. The cardinal process is bifid, stands at an angle of 

 120° with the plane of the valve and is one millimeter in length. On 

 the ventral side of this process is a pit which corresponds to the con- 

 vexity mentioned just in front of the beak on the exterior of the dorsal 

 valve. From the base of the cardinal process five septal ridges 

 radiate. Two of these pass forward at an angle of 25° or 30° to the 

 hinge-line. They also unite back of the cardinal process to form 

 a little lip over the process. Just back and above them are the two 

 sockets for the hinge teeth. 



Two other lateral septa pass forward from the edge of the pit at an 

 angle of 70° with the hinge-line. The fifth ridge is the mesial septum. 

 It is at right angles to the hinge-Hne and extends forward half the 

 length of the shell. With the other four septa it forms the boundaries of 

 the pit. This pit was probably formed at a very young stage, and 



