CRETACEOUS FOSSILS. 273 



(Ostrea vesicularis, d'Orb. (not Lam.); Prod. Pal. Strat., Vol. 2, p. 256, Etage 22, 



No. 925.) 

 (G. dilotata, var. Tucumcarii, Marcou; Bull. Soc. Geol. de France, Vol. xi, pi. 21, 



fig- 3.) 

 (Id., Marcou; Geol. N. A., p. 43, pi. 4, fig. 1, a, b, 2, 3.) 

 (G. Pitcherii, Gabb ; see remarks Proc. Phil. Acad., 1861, p. 22.) 



Shell moderately thick, more or less rounded trigonal, ex- 

 panded. Lower valve variably and unequally bilobate, the 

 large lobe forming about two-thirds of the bulk of the valve, 

 separated from the smaller lobe by a radiating depression, most 

 strongly marked towards the base. Beaks small, incurved, or 

 very slightly deflected towards the side of the small lobe; um- 

 bones large, prominent, and rounded; no umbonal ridge. Sides 

 sloping nearly straight, or slightly convex, downwards from the 

 hinge margin, unequal, the side of the small lobe being always 

 the most prominent. Basal margin more or less strongly sinu- 

 ous. Hinge triangular, short, broad, and straight. Upper valve 

 nearly flat, or a little convex near its straightest margin, caused 

 by a greater thickness at that point; rounded, trigonal to sub- 

 ovate, or almost subcircular. Surface smooth or slightly imbri- 

 cated. 



This description is based principally on Dr. Morton's original specimens, some- 

 what amplified by the assistance of additional material. 



G. navia, Con. 



(GrypJicea Pitcherii, var. navia, Con. ; Emory's Report, Mexican Boundary, p. 155, 



pi. 7, fig. 3, c, d (exclus. a, b).) 

 (G. Pitcherii, Marcou (not Morton) ; Geol. N. A., p. 38, pi. 4, fig. 5, a, b, fig. 6.) 

 (G. navia, Gabb; Proc. Phil. Acad., 1861, p. 22.) 



Shell very thick, elongate, narrow subtrigonal. Lower valve 

 thick, very indistinctly or not at all bilobate ; beaks acute, promi- 

 nent, flattened laterally,and deflected entirely beyond the left hand 

 margin ; umbones prominent, angulated, a sharp umbonal ridge 

 running from the beak to the basal margin; sometimes a slightly 

 marked, rounded ridge occupies the position of the smaller 



PAL. VOL. II. — 36 



