452 CRETACEOUS PALEONTOLOGY. 



usually inconspciuous. Upper valve nearly flat or slightly con- 

 cave. The dimensions of a rather large convex valve are: 

 length, 105 mm. ; height, 98 mm. ; convexity, 58 mm. 



Remarks. It is this species of Gryphaca which makes up in 

 large part, the conspicuous shell bed in the midst of the Nave- 

 sink marl, and it is rarely or never associated in this horizon with 

 any other member of the genus. The species is especially char- 

 acterized by its strongly convex lower valve, the anterior posi- 

 tion of the beak, the conspicuous posterior auriculation and the 

 more or less rugose surface. The shells are extremely variable 

 in general form and outline, as is the case with all species of this 

 group, but the characters mentioned above, although variable in 

 degree of development and in minor details, are apparently con- 

 stant, and there is rarely any difficulty in separating this form 

 from the others recognized in this report. 



Formation and locality. Marshalltown clay-marl, near 

 Swedesboro (180); Navesink marl, Middletown (ii3 2 ), near 

 Crawfords Corner (i26 7 ), near Holmdel (i28 3 , I28 5 , 127, 194),. 

 Marlboro (131), near Walnford (i48 2 ), Crosswicks Creek (149, 

 I47 2 , I47 4 , 195), near Jacobstown (150), Mullica Hill (i6~9 2 ). 



Geographic distribution. New Jersey, Alabama, Mississippi. 



Gryphaea mutabilis Morton. 

 Plate XLVL, Fig. i. 



1828. Gryphaa mutabilis Mort, Jour. Acad. Nat. Sci. Phil., ist 



ser., vol. 6, p. 81, pi. 4, fig. 4. 

 1830. Gryphaa mutabilis Mort., Am. Jour. Sci., ist ser., vol.. 



17, p. 283. 

 1834. Gryphaa mutabilis "Mort., Synop. Org. Rem. Cret. Gr. 



U. S., p. 53, pl. 4, fig- 3- 

 1884. Gryphcca vesicularis White (in part), 4th Ann. Rep. U. 



S. G. S., p. 303, pl. 48, figs. 1-5. 

 1886. Gryphaa vesicularis var. mutabilis Whitf., Pal. N. J., vol. 



i (Monog. U. S. G. S., vol. 9), p. 38, pl. 3, fig. 16; 



pl. 4, fig. 3 ; pl. 5, figs. 1-3. 

 1905. Gryphad mutabilis Johns., Proc. Acad. Nat. Sci. PhiL 



(1905), p. n. 



