1856.] 115 



no fossils ill these beds, but were at that time impressed with the opinion that 

 they belonged to the lower part of No. 1, which is well exposed a little higher 

 up the river at the mouth of the Big Sioux, but soon dips beneath the water 

 level to be seen no more between there and the far distant point already men- 

 tioned, near the mouth of Muscleshell river.* 



DESCRIPTIONS OF SPECIES. 



CyCLAS FORMOSA. 



Shell small, oval, oblique, scarcely ventricose ; cardinal margin straight 

 buccal end rounded ; anal extremity obliquely truncate ; basal margin semi-el- 

 liptical or broadly rounded ; beaks obtuse, tumid, rising somewhat above the 

 hinge, nearly touching, placed a little in advance of the middle ; surface orna- 

 mented by very fine, regular, distinct, concentric wrinkles. Length .11 inch ; 

 breadth .08 inch ; height .14 inch. 



Locality. Three miles above Fort Union. 



Cyclas pragilis. 



Shell small, subcircular, slightly oblique, scarcely ventricose, very thin and 

 fragile ; extremities and base rounded ; posterior end wider than the anterior ; 

 beaks moderately elevated, tumid, slightly in advance of the centre ; surface ap- 

 parently marked with fine indistinct lines of growth. Length .24 inch ; breadth 

 about .13 inch ; height .22 inch. 



All our specimens of this species being more or less worn, it is possible the 

 lines of growth may be more distinct on perfect specimens. Sometimes the 

 posterior slope, from a little behind the beaks, appear to have been obliquely 

 subtruncate. 



Locality. Same as last. 



Cyclas subellipticds. 



Shell small, elliptical-ovate, somewhat ventricose, thin and fragile ; posterior 

 end narrower than the anterior, both narrowly rounded ; base semi-elliptical or 

 semi-ovate ; cardinal border apparently rounding gradually to both extremities ; 

 beaks not much elevated, pointed, incurved, not oblique, located near the 

 middle ; surface indistinctly marked with lines of growth. Length -24 inch ; 

 height -14 inch. 



The beaks are so near the middle, and curved so nearly at right angles to the 

 longitudinal diameter of the shell, that it is not easy to determine, especially 

 from the examination of mutilated specimens, which is the posterior, or which 

 the anterior end. As we have only seen imperfect specimens, we are not sure 

 the surface markings are indistinct on unworn shells. 



Ijocality. Three miles above Fort Union. 



Cyrena Moreauensis. 



Shell ovate, nearly elliptical, compressed, extremities rounded; anterior end 

 narrower than the posterior, base semi-ovate, most promincntbehind the middle; 

 beaks not much elevated, placed a little in advance of the centre ; surface marked 

 with fine distinct lines of growth ; cardinal edge rather thick, and having under 

 the beaks three diverging central teeth in each valve, the anterior of which is the 

 smallest ; lateral teeth two, (in the left valve) long, parallel to the cardinal edge, 

 and fitting into corresponding grooves in the other valve ; muscular impressions 

 deep. Length about -90 inch ; breadth -36 inch ; height -66 inch. 



Each of the cardinal teeth has, in its upper end, a small notch which is occu- 

 pied, when the valves are closed, by a small projection between the teeth of the 

 other valve. The anterior lateral tooth appears to be larger and approaches the 

 central teeth more nearly than the posterior. Our specimens are generally more 

 or less worn, and thickly coated with firmly adhering sand. 



* The foregoing remarks are based upon the observations and collections of 

 Dr. Hayden. 



