MOLLUSCA. 445 



i860. Ostrea falcata Owen, 2nd Rep. Geol. Recon. Ark., pi. 7, 



%• 5- 

 1 861. Ostrea falcata Gabb, Synop. Moll. Cret. Form. p. 208 



(152). 

 1868. Ostrea larva Cook, Geol. N. J., p. 375, fig. 

 1884. Ostrea (Alectryonia) larva White (in part), 4th Ann. 



Rep. U. S. G. S., p. 296, pi. 42, fig. 8. 

 1886. Ostrea larva Whitf. (in part), Pal. N. J., vol. i (Monog. 



U. S. G. S., vol. 9), p. 34, pi. 3, figs. 3-7. 

 1905. Ostrea falcata Johns., Proc. Acad. Nat. Sci. Phil. (1905), 



p. II. 



Description. — Shell of medium size, laterally arcuate. The 

 dimensions of an average specimen are : length along the arcuate 

 median line from beak tO' posterior extremity, 47 mm. ; distance 

 between beak and posterior extremity, 28 mm.; width of shell 

 at middle, 16 mm. ; length oi hinge-line, 20 mm. Shell usually 

 more or less strongly auriculate, the ears subequal or with one 

 ear somewhat larger than the other. Hinge-line straight. Shell 

 marked with from seven to ten deep plications which originate 

 along the lower or convex margin and extend nearly tO' the beak, 

 not leaving a conspicuous non-plicate central area, the plications 

 towards the anterior hinge extremity decreasing regularly in 

 size; along the upper or concave margin the shell is marked by 

 a series, of short, marginal plications. Lower valve moderately 

 convex, with a small scar of attachment; upper valve much flat- 

 ter, its plications similar to those of the lower valve. 



Remarks. — This species, as observed in New Jersey, is most 

 abundantly represented in the Marshalltown marl, where it some- 

 times occurs in innumerable individuals. In its laterally arcuate 

 form it resembles O. mesenterica, but it dififers from that species 

 in its larger size, and in its more strongly plicated shell, there 

 being no central non-plicate area as in that species. The speci- 

 mens vary greatly in the form and size of the auriculations, and 

 to some extent in the number of plications, but not so much in 

 this latter respect as does O. mesenterica. Both of these species 

 have usually been considered as members of a single species, 0. 



