1856.] 275 



very slightly oblique ; surface ornamented with numerous simple, slender, ra- 

 diating costce, alternating with rows of elevated points of about their own 

 breadth; border crenulated? Length -57 inch; height -Sir inch; breadth -48 

 inch. 



In different conditions of weathering the surface markings of this species 

 present a variety of aspects. In some cases the elevated points look as though 

 arranged on costae, instead of between them, while in other instances only the 

 costse are to be seen. Usually, however, these elevated points are quite distinct 

 on casts, and so regularly disposed that worn fragments, when seen embedded 

 in the matrix, present much the appearance of bits of Fenestella, On some 

 specimens traces of fine concentric stri are visible, curving strongly up- 

 wards in crossing the ribs. The shell is so nearly equilateral, and the beaks so 

 slightly oblique, that when viewed on the side it looks much like a BracMopod. 

 We know of no species with which this is liable to be confounded, amongst all 

 the Nebraska shells, and it differs from the allied forms we have seen figured 

 from foreign localities, in having the rows of elevated points of uniform size, 

 and not alternately larger and smaller. 



Locality and position. Bad Lands of Judith River, from a sandstone, the posi- 

 tion of which is doubtful, but probably No. 1 of the series. 



NUCULA OBSOLETASTRIATA. 



Shell thick, oval ovate or elliptical, moderately compressed ; surface orna- 

 mented with fine concentric strife and obscure traces of small closely arranged 

 radiating lines ; buccal end obliquely truncated from the beaks forward to a 

 point nearly half way down the front border, where it is narrowly rounded ; 

 posterior end more broadly rounded ; base semiovate, most prominent behind 

 the center; cardinal border sloping from the beaks at an angle of about 120*'. 

 Beaks small, rather depressed, nearly touching, and placed a little in advance 

 of the middle; lunule narrow ovate, deeply impressed. Length 1-20 inch; 

 height -81 inch ; breadth -60 inch. 



The cardinal edge is thinnest near the beaks, from which point it gradually 

 thickens towards the extremities, especially towards the posterior end. We 

 have had no opportunity of ascertaining whether or not the edge of the lower 

 border is crenulated. As the radiating striae are very obscure, it is probable 

 they are obsolete on worn specimens, though on the surfaces of internal laminae 

 they are usually quite distinct. The anterior muscular impression is deeply 

 impressed, of a narrow ovate form, and placed near the edge of the buccal ex- 

 tremity ; while the posterior one is a little narrower, less deeply impressed, and 

 located close up under the back end of the hinge. There are about thirty hinge 

 teeth in each valve behind, and nearly half as many before the beaks, all of 

 which are curved in crossing the edge, so as to present the concave sides to- 

 wards the extremities. 



This species will be easily distinguished from all the others known to us from 

 the Nebraska formations, by its larger size and other obvious characters. 



Locality and position. Great Bend of the Missouri, No. 4 of the series. 



CuCULLiEA EXIGUA. 



Shell thick, very small, oval cordiform ; posterior end obliquely truncate; 

 anterior extremity rounded ; base nearly straight ; ligament area rather short 

 and straight, marked with longitudinal lines, and in the middle with exceedingly, 

 small, regular, closely set transverse striae. Umbonal region gibbous. Beaks 

 elevated, incurved, rather distant, and located a little in advance of the center. 

 Surface ornamented by irregular concentric wrinkles, and very fine lines of 

 growth, crossed by small, regular, faint, closely arranged radiating striae. Length 

 30 inch ; breadth 27 inch ; height 2'7 inch. 



The ligament area, which is not much more than half the entire length of the 

 shell, is often bounded by a slightly elevated border. That portion of it marked 

 with transverse striae is margined by a fine impressed hair line, and has in each 

 valve a broad triangular form. The longest sides of these two triangles are 



21 



