514 CRETACEOUS PAI^EONTO'LOGY. 



1834. Pkoladomya occidentalis Mort., Synop. Org. Rem. Cret. 



Gr. U. S., p. 68, pi. 8, fig. 3. 

 i860. Fholado-mya occidentdis Con., Jour. Acad. Nat. Sci. 



Phil., 2nd sen, vol. 4, p. 276. 

 i860. Pkoladomya occidentalis Owen, 2nd Rep. Geol. Recon. 



Ark., pi. 8, fig. 9. 

 1861. Phokudomya occidentalis Gabb, Synop. Moll. Cret. Form., 



p. 221 (165). 

 1864. Pkoladomya occidentalis Meek, Check L4St Inv. Foss. 



N. A., Cret. and Jur., p. 14. 

 1868. Pholadom.ya occidentalis Con., Cook's Geol. N. J., p. 727. 

 1886. Pkoladomya occidentalis Whitf., Pal. N. J., vol. i 



(Monog. U. S. G. S., vol. 9), P- i75, pl- 24, figs. 1-3. 

 1905. Pkoladomya occidentalis Johns., Proc. Acad. Nat. Sci. 



Phil. (1905), p. 13. 



Description. — The dimensions of an average sized specimen 

 are : length, about 70 mm. ; height, 47 mm. ; thickness, 45 mm. 

 Shell subovate or subelliptical in lateral outline, and cordate from 

 in front. Hinge-line straight, about two-thirds asi long as the 

 shell; anterior margin rounding from the cardinal extremity 

 intO' the basal margin, or obliquely subtruncate; basal margin 

 gently convex, becoming straighter posteriorly; posterior mar- 

 gin more narrowly rounded than the anterior. Beaks large and 

 broad, situated from one-fifth toi one-fourth the length of the 

 shell from the anterior extremity, strongly incurved and nearly 

 in contact, moderately elevated above the hinge-line. Valves 

 most prominent at about their mid-height in front of the middle 

 of the shell; from this point the surface curves rather abruptly 

 to the ventral anterior and cardinal margins, and much more 

 gently tO' the gaping posterior margin; the cardinal margins 

 back of the beaks are slightly inflected to form ■ a rather distinct, 

 concave cardinal area of moderate width on each valve. Sur- 

 face of each valve marked by 25 to 30 more or less irregular 

 and wavy, rounded, radiating costse of moderate strength, much 

 narrower than the intervening depressions, and closer together 

 in the middle of the shell than at either the anterior or posterior 



