MOLLUSCA. 459 



1853. Exogyra ponder osa Shum., Marcy's Expl. Red. River 



La., p. 204. 

 1870. Exogyra ponder osa Credner, Zeitsch. Deutsch. Geol. 



Ges., Band 22, p. 229. 

 1875. Exogyra ponderosa White, Rep. Geog. and Geol. Expl. 



w. looth, Mer., vol. 4, pi. i, p. 172, pi. 14, figs, i a-c. 

 1884. Exogyra ponderosa White, 4th Ann. Rep. U. S. G. S., 



p. 306, pi. 50, figs. 1-3. 

 1886. Exogyra costata Whitf., Pal. N. J., vol. i (Monog. U. 



S. G. S., vol. 18), p. 39 (in part), pi. 6, figs, 1-2. 

 1902. Exogyra ponderosa Hill and Vaughn, U. S. G. S., Geol. 



Atlas, Austin Folio, fig. 46. 



Description. Shell large, plano-convex, more or less sub- 

 ovate in outline. Lower or right valve strongly convex; beak 

 strongly coiled; the surface more or less subangular along a line 

 extending obliquely from the beak to the postero-ventral margin, 

 the posterior slope convex, the anterior slope flattened towards 

 the beak, irregularly flattened, slightly concave or convex below ; 

 surface marked by more or less distinct, rather broad and flat, 

 more or less irregular, sometimes bifurcating costae on the 

 younger shells and on the umbonal portion of the adult shells, 

 which become obsolete on the outer portion of full grown indi- 

 viduals, giving place to strong, concentric, lamellose extensions 

 of the shell. Upper or left valve nearly flat, coiled at the apex, 

 marked with conspicuous concentric lamellae and by a few more 

 or less indistinct radiating costae on the antero-dorsal region. 



The dimensions of a large individual are: height, 123 mm.; 

 width, 114 mm.; convexity, 60 mm. 



Remarks. In the Cretaceous beds of New Jersey this species 

 occurs only in the Marshalltown clay-marl, and it has not been 

 found in association with E. costata, although these shells have 

 usually been included in that species. In fact the illustration 

 in Whitfield's monograph is really a figure of this species rather 

 than of the true E. costata. The species differs from E. costata 

 in the much weaker development of the radiating costae and in 

 the strong development of the concentric lamellae. 



