24 Bulletin 31 34 



ribs of the right valve of the latter are more or less split up, but in P. will- 

 coxi present the appearance of a fascicle of separate threads. In v>'orn spec- 

 imens of ivaiitubbeanus the ribs appear rounded and plain after the removal 

 of the scales ; in willcoxi the division into threads is distinct. Nevertheless 

 it is possible that a larger series may show the two forms to be merel}' the 

 extremes of a single species. From P. inenibranosus the present form is 

 easily distinguished by wider hinge line, larger ears, thinner shell, and by 

 its radial threads fasciculated rather than subequally level. 



We have given Ball's Statements in full regarding this form 

 for but few specimens seem to have been included in any of our 

 collections. The specimens figured were loaned from the U. S. 

 Nat. Museum through Dr. Dall and came from "Clark Co.", 

 Miss. 



Type.—V. S. Nat. Museum. 



Horizon. — St. Maurice Eocene. 



General distrib7ition.~ '''Eocene oi Clark Co., Miss., and of 

 the Wautubbee Hills (Claibornian) ; Johnson and Burns." — 

 Dall. 



Variety cainei n. var., PI. I4i Fig- 8. 



This is characterized b)^ the broad, simple ribs ornamented 

 with densely arranged, imbricate concentric lines. 



Named in honor of T. A. Caine, a former student who col- 

 lected a large amount of material from eastern Miss. 



Type.—C. U. Collection. 



Horizon. — St. Maurice Stage. 



Localities. — Wautubbee and Hickory, Miss. 



Pecten pulchricosta Aid. and Mr,, Pi. 14, Figs. 9. 10 • 



p. pulchricosta. Aid. Mr., Jr. Cinn. Soc. Nat. Hist., vol. 9, 1886, p. 45, 



pi. 2, fig. 23, a. 

 P. pulchricosta Dall, Trans. Wag., 1898, p. 730. 



Aid. and Meyer's original description. — Convex, covered by eight 

 broad, rounded, radiating ribs, perceptible in the inside ; those in the mid- 

 dle are largest. Near the ventral margin they dissolve into more numerous 

 ribs. Wautubbee : only the figured valve is known. 



Dall has increased our knowledge regarding the specific 



characters of this species from National Museum material. He 



says ; 



