MARYLAND GEOLOGICAL SURVEY 609 



Type Locality. New Jersey. 



Shell subcircular or irregular in outline with a silvery sheen both 

 within and without, thin but tough, of moderate size, the adults from 

 15 to 30 mm. in circumference ; left valve usually convex, though varying 

 widely in the degree of convexity; right valve, through which the byssus is 

 extruded, flattened; umbones central, almost marginal, very inconspicu- 

 ous, scarcely interrupting the regular outline of the valve ; external sur- 

 face ornamented with thin, concentric overlapping lamellae which are 

 frequently radially lineated ; ligament submarginal, attached beneath the 

 umbo of the left valve ; hinge edentulous ; interior scarred with a large, 

 major byssal impression, medial in position and quite high up under the 

 umbones and ventral to it, the minor byssal impression, and the posterior 

 muscle adductor ; a third byssal scar of minute size underneath the dorsal 

 margin, a little in front of the umbones ; inner ventral margins simple. 



This species is one of the most abundant bivalves in the Upper Cre- 

 taceous faunas of Maryland. For all the shell is so thin, it is very tena- 

 cious and easily separable from the matrix. It is an unusually well char- 

 acterized species and even the fragments can be determined with assurance 

 by the silvery sheen, the crowded concentric laminae and in the majority of 

 individuals by the fine, radial lineation. 



The form varies to a certain extent, as do all members of this variable 

 genus, in the outline, the degree of compression of the valves, and par- 

 ticularly in the development of the radial sculpture. However, limits must 

 be placed even for variable species and it is not probable that they should 

 be made wide enough to include A. tellinoides Conrad, which is constant 

 in its transversely ovate outline, lack of lustre, rather distant concentric 

 lamination and absence of radial striations. 



Some puzzling little forms from the Monmouth at Brightseat are closely 

 related genetically with the A. argentaria Morton. They are apparently 

 young, frequently ovate, rather thin, circular in outline and are sculptured 

 with a few wide, sharp-edged concentric frills which are often radially 

 lineated. Concentric lamina? so distantly spaced and so sharply frilled 

 have not been observed among the A. argentaria. 



