FROM THE CRETACEOUS FORMATION OF NEBRASKA. 383 



posterior margin regularly rounded ; surface marked by fine equal concentric lines. 

 Length, .18 inch; height, 1 inch; width, .66 inch. 



The form is neatly rounded throughout, the iimbones curving gently towards the 

 antero-cardinal margin. Our specimens of this shell are all imperfect. 



Locality and Position. — On the Missouri, five miles below James River. Calcareous 

 beds of the base of division No. 2 of Section. 



Cttherea tenuis, n. sp. 

 Plate I. Fig. 8, a, t, c. 



Shell thin, ovate-orbicular, length and height nearly equal ; beak elevated, nearly 

 central ; anterior and posterior extremities rounded, the latter somewhat broader ; 

 surface marked by concentric imdulations and fine parallel striae. Length, A inch ; 

 height, .36 inch. 



This is a fragile shell with beaks more nearly central than the preceding species. 

 It is much more delicate than any shell of this family which has been found in the 

 cretaceous formation of this region. 



Both this and the preceding species are referred to the genus Cytherea from external 

 form, no opportunity having oftered of examining the hinge. 



Locality and Position. — Same as preceding. 



Crassatella Evansii, n. sp. 

 Plate I. Fig. 9, a-e. 



Shell obliquely ovoid (varying somewhat in form), ventricose ; beaks much elevated ; 

 anterior margin short, rounded below; postero-cardinal margin sloping abruptly down- 

 wards, the extremity sub-truncate ; basal margin distinctly and neatly crenulated on 

 the interior ; escutcheon broad lanceolate, well defined ; lunule distinct, but margins 

 not strongly defined ; surface somewhat undulated, marked by fine irregular stri?e or 

 lines of growth ; muscular impressions strongly marked. 



This shell is probably identical with the imperfect cast figured by Dr. D. D. Owen 

 in his Eeport, PI. 7, fig. 9, as a Pectunculiis. Our specimens showing the interior of 

 the hinge, muscular impressions, etc., are from the same position in the series, and 

 from the same district of coimtry. The shell is a Avell-marked Crassatella, presenting 

 all the ordinary characteristics of the genus, in the cardinal and muscular characters. It 

 is abundant, occurring entire and in the condition of casts. The species may be readily 



