1856.] 



87 



It is with much doubt we place this shell in the above genus, as we have in 

 none of our specimens seen any indications of an anterior wing. It is also 

 worthy of note, that the substance of the shell is composed of an internal 

 lamellar, and an external fibrous portion, like Finna, from which, however, it 

 differs in having a posterior wing-like expansion. The radiating costae are faint 

 or wanting on the wing. 



Locality and position. Forks of Cheyenne River, No. 4 of series. 



Inoceramus ventricosus. Shell ovate, oval or oblong, extremely inflated, 

 very thin and fragile, structure entirely fibrous ; cardinal border straight ; ante- 

 rior end rounded, posterior end apparently' subtruncate, base broadly curved ; 

 umbonal region remarkably gibbous ; beaks oblique, located at the anterior 

 extremity ; surface marked with fine regular imbricating lines of growth, and 

 occasionally with a few faint irregular con' entric undulations. Length about 

 4-30 inches ; breadth (of right valve) I'TO inches ; height 2-T5 inches. Thickness 

 of thickest part of shell, near the hinge, -12 inch ; do. of thinnest part near border 

 03 of an inch. 



The most striking characteristics of this species are its remarkably ventricose 

 form, extreme thinness and entirely fibrous structure, there being no internal 

 lamellar portion, as is generally the case in this genus. In our specimens the 

 calcareous matter has been replaced by ferruginous and silicious matter, so 

 as to preserve the original structure of the shell perfectly. If the left valve is as 

 gibbous as the right, the transverse diameter of the shell must be considerably 

 greater than its height. The beak of one of our specimens appears to have been 

 truncated. We have only seen right valves. 



Locality and position. Mouth of Judith River, in sandstone, supposed to be the 

 same as No. 1 of the series. 



Pecten Nebrascensi.s. Superior valve suborbicular, moderately compressed, 

 ornamented by twelve to fifteen strongly elevated costae about equal to the 

 spaces between ; surface marked with very fine, closely arranged concentric lines, 

 crossed by equally fine crowded radiating striie ; buccal ear (imperfect in our 

 specimens) apparently triangular, rather distinctly marked with radiating costae, 

 crossed by concentric striie stronger than on the body of th valve ; aunal ear 

 smaller, triangular, concave on the margin, and marked with distinct concen- 

 tric lines. Length and breadth -49 inch. 



From F. venustus of Dr. Morton, (Synopsis, pi. 5, fig. 5,) to which this species 

 appears to be closely allied, it may be distinguished by the fine radiating striae. 

 In some of the specimens the costaj occasionally bifurcate, while the radiating 

 strife never run exactly parallel to the costae, but pass very obliquely along 

 their sides. These striae are so fine as to be invisible without the aid of a lens. 

 We have not seen the inferior valve. 



Locality and position. Yellow-stone River, one hundred and fifty miles from 

 its mouth, in a bed supposed to represent No. 5 of the series. 



Natica subcrassa. Sh^ll obliquely oval or oblong, thick ; spire rather short ; 

 volutions three to three and a half, convex ; suture distinct or somewhat grooved ; 

 surface marked with strong lines of growth, which rise, on the body whorl, into 

 distinct imbricating wrinkles ; outer lip bevelled ; inner lip rather thick, and 

 nearly covering the small umbilicus ; aperture ovate, oval, or elliptical, nearly 

 as obtusely rounded above as below. Length -95 inch ; breadth -87 inch ; apical 

 angle about 93. 



It is possible there may have been other surface markings than those men- 

 tioned above, as all our specimens are so worn that fine lines would have been 

 obliterated, had they existed. The species may be known from those found 

 in the higher members of the series in this region, by its stronger lines of growth, 

 and greater thickness. 



Locality aiid position. Mouth of Judith River, from a sandstone supposed to 

 be the same as No. 1 of the series, where it is associated with Tellina gracilis, 

 and T. equilateralis. 



