232 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



A low, broad, inosiul sinus oecui's on nearl}" all specimens of the 

 dorsal valve, being strongest in the young shells, and occasionally a 

 shallow sinus is clearly defined on the ventral valve. It is quite rare 

 to tind a well-marked median fold on the ventral valve. 



The surface is marked by sharply rounded, line, radiating costse, 

 crossed by tine, concentric lines of growth, and very fine, slightly 

 undulating concentric strite. The size and sharpness of the radiating 

 costaj vary greatly in specimens from the same bed at the typical 

 locality in Texas. Shells occur with strong, rounded costse, and no 

 traces of fine, elevated lines between, while others show from one to 

 four or more lines that start at varying distances from the beak. The 

 imbricating lines of growth give a concentrically ridged aspect to 

 some shells, while others are nearly smooth from beak to frontal 

 margin. The increase in the num])er of costs? is by interpolation, not 

 by bifurcation. 



There is considerable variation in the size of the shell. The largest 

 specimens from Texas have a transverse diameter of 19 mm., with a 

 heighthof 14 mm. for the dorsal valve, and about 18 mm. for the ven- 

 tral valve. Specimens of the ventral valve from the Yellowstone 

 National Park have a height of 14 mm. with a width of 15 mm. 



Cardinal area of ventral valve moderately high. It is marked by 

 transverse stride of growth that cross it parallel to its base and arch 

 over the convex deltidium. The plane of the area extends backward 

 at an angle of about 10*^ to the plane of the margin of the shell. The 

 delthyrium is strong and covered by a convex deltidium, the front 

 margin of which arches back about one-fifth the length of the del- 

 th3a"ium; the deltidium is marked by concentric striae of growth and 

 fine radiating lines and a minute perforation near its apex. The cardi- 

 nal area of the dorsal valve is short as comj)ared with the ventral 

 valve. It extends backward at an angle of about 45° to the plane of 

 the margin of the valve. It is divided midway by a strong delthyrium, 

 which is covered about half its distance by a convex chilidium. 



In the interior of the ventral valve the hinge teeth appear to be sup- 

 ported by dental plates that extend down to the bottom of the valve 

 and bound the tripartite umbonal space opposite the delthyrium, into 

 which the vascular sinuses extend on each side of the diductor muscle 

 impressions, or their path of advance. The traces of the vascular S3^s- 

 tem are confined to the main vascular trunks, which extend forward 

 nearly to the front margin, where in some examples they are bifur- 

 cated. The inner branch extends in toward the median line, disap- 

 pearing in the numerous radiating depressions near the margin. The 

 lateral branches appear to connect with the peripheral canal, that arches 

 about the space, probably occupied by the ovarian areas, between 

 itself and the main vascular trunks. The spaces for the attachment of 

 the muscles between the main vascular trunks appear to have been 



