538 Systematic Paleontology 



Shell small, inflated ; umbones rather high, convex, incurved at the tips 

 and proximate submedial or slightly anterior in position; outline rudely 

 trapezoidal; anterior end broadly rounded or squarely truncate; posterior 

 end slightly produced obliquely; base line nearly horizontal medially, 

 evenly rounded anteriorly, obtusely angulated posteriorly ; posterior area 

 delimited by a ridge running from the umbones to the posterior ventral 

 margin, growing increasingly obscure toward the base ; hinge line straight, 

 approximately three-fourths of the total length of the shell; hinge area 

 quite low, rhomboidal in double valves ; ligament confined to a smaller 

 rhomb below the umbones, outlined by a rather deep linear sulcation 

 transversely striated in slightly weathered individuals ; external sculpture 

 delicately reticulate over the entire surface with the exception of the ante- 

 rior and posterior submargins ; radial lira? very fine, particularly upon the 

 posterior slope, tending to alternate in strength upon the medial area 

 and anterior slope, numbering between forty and fifty in all; concentric 

 lirations broader and flatter than the radial threadlets which they over- 

 run; inters] tira Is linear in the umbonal region, less narrow away from the 

 umbones, thus making the interstices between the reticulate lira? more 

 squarish in outline toward the base; hinge plate slightly arcuate ven- 

 trally ; hinge teeth set in a single series, very short and straight beneath 

 the umbones, but becoming longer and more oblique distally ; number 

 usually twenty-one to twenty-three in all, those behind the umbones 

 exceeding by one or two those in front of them ; muscle scars well defined, 

 the posterior outlined in part by the elevated ridge in front of it ; pallial 

 line distinct, very close to the ventral margin. 



Area saffordi Gabb has been discussed chiefly from casts, so that the 

 diagnostic characters of the surface sculpture have not been emphasized. 

 Gabb mentions the presence of a concentric sculpture in his original 

 description, but it seems to have been disregarded by the subsequent New 

 Jersey paleontologists. A rca ereineca Conrad is, apparently, more rounded 

 in outline, the radial sculpture is coarser and dominates the concentric, 

 and the hinge teeth are fewer and more oblique. A. saffordi Gabb is the 

 analogue in the East Coast faunas of ,4. exujua Meek and Hayden of the 



