MOLLUSCA. 453 



Description.^Shell more or less subcircular in outline, the beak 

 subcentral, the hinge-line usually nearly straight. The lower 

 valve depressed convex usually somewhat compressed towards 

 the cardinal extremities, the posterior constriction and auricula- 

 tion obsolete or scarcely distinguishable. The upper valve nearly 

 flat or moderately concave. Surface of the shell marked with 

 concentric, more or less squamose lines of growth. Shell sub- 

 stance moderately thick. Scar of attachment small or of moderate 

 size. 



The dimensions of an ordinary lower valve are: length, no 

 mm. ; height, 96 mm. ; convexity, 33 mm. 



Remarks. — This form is especially characteristic of the Mar- 

 shalltown marl, and it differs from G. convexa of the Nave- 

 sink, in its more depressed convex lower valve, in its thinner 

 shell and in the absence of the conspicuous posterior constriction 

 and auriculation of the lower valve. The two species are some- 

 times associated in the Marshalltown formation, but specimens 

 are rarely met with which cannot be placed without hesitation irt 

 either the one or the other species. In the Navesink marl this 

 species is rarely or never met with 



Pormation and locality. — Marshalltown clay-marl, near 

 Swedesboro (177, 179, 180). 



Geographic distribution. — ^New Jersey. 



Gryphaea dissimilaris n. sp. 



Plate XLVL, Figs. 2-3. 



Description.- — Shell thin or oi moderate thickness, somewhat 

 oblique, variable in outline, but usually more or less subovate, 

 usually wider than long, the beak subcentral or in front of the cen- 

 ter. Lower valve usually strongly convex, with the beak 

 incurved, often with a slight or moderate constriction passing 

 from the posterior side of the beak to the posterior margin, 

 limiting a more or less prominent posterior auriculation ; in many 

 specimens this constriction and consequent auriculation is entirely 

 obsolete. Upper valve smaller, often very much smaller • than 

 the lower, deeply concave as a whole, although the portion near 



