INVERTEEIiATE PALyEONTOLOOY. 



115 



Length, 4.10 inches; height, 2.3<i inches; convexity, 2.07 inclies. 



This shell has SO much the aspect (if the existing Uiliones that I referred 

 it provisionally to that genus in first describing it, placing the name Baphia 

 (which has been by some considered synonymous with Margaritand) paren- 

 thetically between the specific and generic names, with a mark of doubt, 

 to indicate that I thought it might belong more properly to the latter. It 

 certainly has an external ligament, and all of the external characters oi 

 Unio and Margaritdna ; but so far as can be determined from internal casts, 

 it almost certainly has no posterior teeth, and in this character does not agree 

 with Unio. One cast shows the impression of a single compressed anterior 

 tooth, ranging parallel to the anterior slope, just above the anterior muscular 

 impressions and near the margins. These muscular scars are deep, and like 

 those of Unio and Margaritana, though I am not quite sure of the presence 

 of the pedal scars just behind those of the anterior adductors, as we see in the 

 genera mentioned, as well as in other types of the Unionidce, unless the}' may 

 be represented by the little protuberance seen projecting from the posterior 

 side of one of these scars, as represented in our figure 5, c. There are 

 certainly no traces of pedal scars above the anterior adductors, as we see in 

 the CrassatdlidoB and other families to which our shell would most probably 

 belong, if it does not fall into some genus of the Unionidce. The casts do 

 not show the pallial line, nor the impressions of the posterior adductor muscles. 



Locality and position. — Dakota group (No. 1) of the Upper Missouri 

 Cretaceous series. Opposite Sioux City, on the Missouri River, in Dakota 

 County, Nebraska, where it occurs associated with Cyrena Dakotensis. 



CRASSATELLID/E. 



Genus CEASSATELLA, Lamarck. 



Synon.—CrassaleUa, Lamarck (1801), Syst. An., 119; and (1609), Phil. Zool., *; also (1818), Hist. Nat., V, 

 480.— Blainv. (1818), Diet. Sci. Sat., XI, 356.— Desh. (1830), Encyc. Moth., Ill, 19.— 

 Broun (1838), Leth., 971.— G. B. Sowerby, jr. (1842), Couch. Man., 2d ed., 125.— Nyst, 

 (1846), Coq. Tert. Belg., S3.— Gray (1847), Proc. Zool. Soc. Loud., 194.— Woodward 

 (1856), Man. Moll., 299.— H. and A. Adams (1857), Gen. Recent Moll., II, 485 ; aud many 

 others (not Lamarck, 1799, = Mactra, Liun.). 

 Pachythcervs, Conrad (1870), Am. Jour. Couch., V, 47; and (1872), Proceed. Acad. Nat. Sci. 

 Philad., XIV, 50; also (1873), Kerr's Geol. Rep. S. Car., App., 6 (as a subgenus of 

 Crassatella). 



Etym. — Crassus, thick. 



Type.— C. gibba (= Venus ponderosus, Chemnitz). 



Shell transversely oval-oblong or subtrigonal, more or less convex, 

 usually thick and strong, iuecpiilateral, often somewhat attenuate posteriorly; 







