370 CRETACEOUS PALEONTOLOGY. 



1886. Nucula percrassa Whitf., Pal. N. J., vol. i (Monog. U, 

 S. G. S., vol. 9), p. 102, pi. II, fig-s. 4-6. 



1886. Nucula slackiana Whitf., Pal. N. J., vol. i (Monog. U. 

 S. G. S., vol 9), p. 103, pi. II, figs. 2-3. 



1886. Donaxfordii Whitf., Pal. N. J., vol i (Monog. U. S. G. 

 S., vol. 9), p. 171, pi. 23, fig. I. 



1905. Nucida percrassa Johns., Proc. Acad. Nat. Sci. PhiL 



(1905), P- 7- 

 1905. Nucula slackiana Johns., Proc. Acad. Nat. Sci. PhiL 



(1905), p. 8. 

 1905. Donax fordii Johns., Proc. Acad. Nat. Sci. Phil. (1905), 



p. 17. 



Description. — Shell moderately convex, attaining a large size 

 for the genus, the largest New Jersey specimen observed being: 

 length, 32 mm., and height about two-thirds the length. Shell 

 subelliptical in outline, the beaks situated at about the anterior 

 third of the shell, the greatest length of the shell at about the 

 mid-height. Anterior margin obliquely subtruncate above, the 

 anterior extremity of the shell subangular ; basal margin between 

 the two extremities of the shell usually regularly rounded, some- 

 times more or less obscurely obliquely subtruncate in front and 

 sometimes somewhat straightened ventrally; posterior margin 

 more or less sharply rounded; the postero-dorsal margin gently 

 convex or nearly straight. Valves regularly convex, antero- 

 dorsal slope rather abrupt, passing into the rather large lunular 

 depression. Surface of the shell marked by more or less irregu- 

 lar concentric lines of growth, and by fine, regular, radiating 

 costae, narrower than the interspaces, which are more strongly 

 developed on the anterior portion of the shell. The shell sub- 

 stance thick. Teeth strong, about 20 posterior and 8 or 10 

 anterior to the beak, both series diminishing in size as they ap- 

 proach the beak. Well preserved internal casts preserve strongly 

 defined muscular impressions and pallial line, and are strongly 

 crenate about the free margin. 



Remarks. — This species occurs more or less commonly in sev- 

 eral of the Cretaceous formations of New Jersey, and presents 

 several different aspects because of the different modes of preser- 



