FROM THE CRETACEOUS FORMATION OF NEBRASKA. 385 



tance from the present one ; showing that the shell increased by additions to its car- 

 dinal, as well as ventral margin. 



Locality and Position. — Sage Creek. Upper part of division No. 4 of Section. 



NUCULA VENTRICOSA, H. SJ). 

 Plate I. Fig. 11, a, b. 



Shell ventricose in the middle and depressed at each end, ovate, prolonged posteriorly, 

 somewhat regularly rounded in front, contracted behind ; a broad shallow impression 

 extending from below the beak to the postero-ventral margin of the shell ; beaks ele- 

 vated, nearer to the anterior extremity ; basal margin regularly rounded to near the 

 posterior end ; surface marked by regular, distinct concentric ridges, which are strong 

 upon the middle of the shell, and become somewhat abruptly obsolete as they pass to 

 the depressed parts at either extremity; hinge line slightly curved; crenulations 

 strong. Length, .14 inch ; height, .09 inch ; width about the same as height. 



This minute shell is distinguished by its ventricose middle and depressed extremities, 

 and by the strong concentric ridges becoming almost abruptly obsolete in passing from 

 the ventricose portion of the shell to the flatter extremities ; the teeth of the hinge line 

 are comparatively strong. The mature character of the shell is indicated by its thick- 

 ness, as well as by the strong concentric ridges ; while several casts of the same dimen- 

 sions were found associated with it. 



Locality and Position, — Sage Creek. In division No. 4 of Section. 



Captjltjs occidentalis, n. sp. 



Plate I. Fig. 13, a-rf. 



OrUcula (undet), Owen, Report. PI. vu. Fig. 11. 



Sub-orbicular, patelliform ; base nearly flat ; very depressed, conical above ; the apex 

 intermediate between the centre and margin ; lower surface marked by fine lamellose 

 imbricating radiations, which diverge from a point nearly opposite the apex of the 

 convex side, and are crossed by concentric undulations. The convex side is distinctly 

 marked by a horseshoe-form muscular impression, which is connected at its two 

 extremities by a fainter parallel impression ; muscular impression marked transversely 

 by radiating striae, which are continued obscurely beyond it to the margin, and which, 

 in the muscular impression, are crossed by finer concentric lines. 



VOL. V. NEW SERIES. 52 



