3/o CRETACEOUS PALEONTOLOGY. 



1886. Nuciila percrassa Whitf., Pal. N. J., vol. i (Monog. IL 

 S. G. $., vol. 9), p. 1 02, pi. n, figs. 4-6. 



1886. Nuciila slackimia Whitf., Pal. N. J., vol. i (Monog. U. 

 S. G. S., vol 9), p. 103, pi. n, 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. Doiiax 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- 



