652 SYSTEMATIC PALEONTOLOGY 



Crassatella subplana Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, p. 



121, pi. xviii, figs. 14-16. 



Crassatellites subplanus Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 14. 

 Crassatellites subplanus Weller, 1907, Geol. Survey of New Jersey, Pal., 



vol. iv, p. 553 (ex parte, synonymy excluded.) 



Description. " Subtriangular, compressed or plano-convex ; anterior 

 margin obtusely rounded ; posterior extremity subtruncated ; posterior 

 basal margin straight or slightly contracted ; disk marked with numerous 

 prominent acute concentric ridges and fine concentric lines." Conrad, 

 1853. 



Type Locality. Arneytown, New Jersey. 



" The dimensions of a small specimen, a nearly perfect right valve, are : 

 Length 36 mm., height 28 mm., convexity 6 mm. Large individuals grow 

 to a length of 50 mm. or more. Shell broadly subovate in outline, beak 

 obtuse, situated about one-third the length of the shell from the anterior 

 extremity. Antero-cardinal margin straight or slightly concave, sloping 

 downward from the beak ; anterior margin rounding into the basal margin, 

 moderately convex throughout to the postero-basal extremity, which is 

 obtusely subangular ; posterior margin short, truncated nearly vertically or 

 slightly inclined; postero-cardinal margin gently convex, sloping down- 

 ward from the beak and meeting the posterior margin in an obtuse angle. 

 Surface of the shell with an obtusely angular umbonal ridge, which passes 

 from the beak to the postero-basal angle in nearly a straight line, the post- 

 cardinal slope slightly concave to the cardinal margin ; the post-cardinal 

 margin sharply inflected to form a rather deeply excavated escutcheon ; 

 antero-cardinal margin inflected to form a deep but rather ill-defined 

 lunule. Surface of the shell marked by regular, somewhat imbricating, 

 concentric lines of growth, and often by a few broader concentric undula- 

 tions towards the margin. Hinge of the right valve with a strong car- 

 dinal tooth transversely striate on its anterior surface, directly beneath the 

 beak. Behind it is a very large and broad triangular pit, with a much 

 smaller secondary pit just behind the lower end of the tooth ; in front of 

 the cardinal tooth is a small triangular pit about equal in size to the 

 secondary pit behind, and in front of this pit a low, obscure, tooth-like 

 ridge extends obliquely forward to the upper margin of the anterior mus- 



