38 PALEONTOLOGY OF NEW JERSEY. 



regularly rounded on the back ; that is, without the subangular character. 

 It is extremely abundant in the middle marls of New Jersey, where it is 

 almost without any admixture of other forms, and I have seen beds nearly 

 eight feet in thickness composed almost entirely of this shell and Terebratida 

 Harlani. They are not at all uncommon in the Lower Green Marls at many 

 localities. 



G, Pitcheri var. navia. This shell is small, elongate ovate in outline, 

 widening below with a strongly incurved beak, or broad truncation of that 

 of the lower valve, highly concavo-convex with a short hinge line, and des- 

 titute of an}' or with scarcely perceptible constriction. Resembles G. in- 

 curva from the Jura, but is less incurved at the beak. 



Var. nmtabUis Morton. (Syn. Org. Rem., p. 63. PI. IV, Fig. 3 (=^Pijc)io- 

 donta mutabilis). Shell broadly depressed-convex on the lower side and flat 

 or slightly concave on the upper. Outline more or less circular, with a mod- 

 erately extended hinge line, which is often more or less arcuate on the 

 outer margin ; and with a broad ligamental area. Posterior constriction of 

 the lower valve obsolete or barely distinguishable, not marking the upper 

 valve. 



Var. No. 4 (see PI. Ill, Fig. 15). Somewhat resembling the last, but less 

 regular in outline and with a straighter hinge line, but with a deep constric- 

 tion which appears at a late stage of growth and modifies the ujiper valvt 

 to a great degree. It is placed at a considerable distance below the cardinal 

 line, and is deeply bent backwards to nearly the depth of the convex or 

 lower valve. 



Of course there are many intermediate forms found scattered through 

 the marls at various elevations and at different localities, which would readily 

 serve to unite all these forms, and especially is this the case if we take into 

 consideration those from other parts of the country and from Europe, but 

 there are man}' localities where among thousands of individuals, of different 

 stages of growth, pertaining to one of these varieties, one will search long 

 before finding forms enough to satisfactorily link the large ponderous shells 

 with the small highh' arcuate but adult shells of the varieties navicella and 

 navia. 



Formation and localities. — Among the specimens which I have examined 

 in the various cabinets, I have seen them from nearly all of the noted locali- 



