200 AMERICAN JOURNAL 



from the first in outline ; it is longer, is produced in advance, 

 the posterior end is more produced and narrow, the base is ir- 

 regularly convex, most prominent in the middle ; the beak is in- 

 curved, but is much less spiral than the left valves of the smaller 

 species ; under the beak is a large lunular depression, bordered 

 by a ridge ; near this, running to the anterior basal margin, are 

 two faint radiating ridges ; posteriorly there is a very indistinct 

 umbonal ridge. The same production of the anterior margin, 

 and its truncation above, that occurs in the preceding species, 

 exists in this also, but in a less degree, and the truncation is 

 oblique. The hinge is smaller than in obliqua, the pit is propor- 

 tionally broader and shorter ; the posterior lateral tooth is well 

 marked. Anterior muscular scar small, deep ; posterior, large, 

 subquadrate aud impressed ; pallial line strongly marked, near 

 the margin, and suddenly bent upwards for a very short distance 

 posteriorly. 



Dimensions. — Length, '85 in., width, -59 in., height, *23 in. 



