6 PROCEEDINGS OF THE NATIONAL MUSEUM vol.71 



dorsal margins. In all other directions the slope is more gradual. 

 As shown in an anterior view, the anterior portion of the shell is 

 decidedly flattened. Along the line of contact of the valves is a 

 well-marked depression due to a decided incurving of the valves 

 along their anterior margins. 



The shell itself is very thick, especially in the umbonal region and 

 along the line of greatest arching. In the umbonal region of large 

 individuals the shell may be 5 centimeters (2 inches) or more in 

 thickness, while specimens of medium size have a thickness of an 

 inch or more in this part of the shell. The shell becomes thinner 

 toward the margins but at all times is heavy. The surface of the 

 shell is marked by fine concentric lines, which become much coarser 

 in the flattened anterior portion of the valves. An inconspicuous 

 lunule and a large, well-defined escutcheon are present. 



The hinge plate is very broad in the anterior portion of the 

 valve, narrowing posteriorly and then becoming wider again. In 

 its posterior part it flexes abruptly downward. The anterior teeth 

 are five or six in number in each valve and are usually subequal in 

 size. They are long, nearly straight, narrow and parallel. They 

 lie at nearly right angles to the hinge or slope somewhat posteri- 

 orly. Rarely two of the teeth partially fuse laterally, giving the 

 effect of fewer, more massive teeth, as shown in Plate 1, Figure 4. 

 This appears to be an individual variation and is only partially 

 to be correlated with ontogenetic change. In the posterior portion 

 of the hinge plate is a variable development of posterior-lateral 

 teeth. Usually there is one well-defined tooth which shows as a 

 low narrow ridge. This may start near the group of anterior teeth 

 or much farther back on the hinge plate. Always it appears to be 

 present in the downward flexed portion of the hinge plate. In addi- 

 tion to this long narrow posterior-lateral there may or may not be 

 present one or more linear, short, more massive teeth, usually re- 

 stricted to the downward flexed portion of the plate but occasionally 

 anterior to the flexure. In addition to these fairly well-defined 

 teeth there is a variable development of amorphous elevations and 

 depressions posterior to the group of anterior teeth. 



The anterior muscle scar lies at the lower anterior angle of the 

 group of anterior teeth. In all but one specimen seen the scar lies 

 in a small inconspicuous pit. In this one exception (pi. 2, fig. 5) the 

 muscle scar is far larger than in any other specimen observed, ir- 

 respective of size, and may be an individual abnormality. Possibly 

 the sj)ecimen is referable to another species. It is to be noted that 

 the specimen is referred to this species with a question, as it has other 

 characters, such as a longer hinge line, more projecting anterior 

 portion of the shell, and more pronounced surface sculpture than 



