300 GEOLOGICAL SURVEY OF THE TERRITORIES. 



cartilage processes, in the casts of Udmonclia, though I am not quite 

 sure that it belongs properly to the latter genus. It is a rather thin 

 shell, that leaves impressions of the concentric ridges rather distinct on 

 internal casts, where very faint traces of radiating lines are also some- 

 times seen. Its muscular and pallia! impressions are obscure, and not 

 well known. 



Locality and position. This shell has a very wide distribution in the 

 coal measures of this country, through nearly the entire thickness of which 

 it ranges. The typical specimen was found in the upper part of the coal 

 measures at Aspenwall, on the Missouri River, in Southeastern Nebraska. 

 It also occurs at nearly all the other outcrops along the Missouri in that 

 region, as well as in the upper and lower part of the coal measures of 

 Illinois ) likewise in the lower part of the seres in West Virginia, and 

 apparently at Box Elder Canon, on the North Platte, Wyoming. The 

 specimens from the latter locality, however, are shorter and more gib- 

 bous, and agree more nearly with Professor Geinitz's figure.' 



Cretaceous forms. 



genus CRASSATELLLNA, (meek.) 



Shell subtrapezoidal, equivalve, inequilateral, with the margins closed 

 and smooth within. Hinge with two cardial teeth and one elongated 

 anterior and posterior lateral tooth in each valve. Anterior cardinal 

 tooth of left valve trigonal and deeply emarginate below ) posterior very 

 oblique, and separated from the other by an oblique pit divided longi- 

 tudinally by a thin lamina. Cardinal teeth of right valve diverging, 

 with a triangular pit between for the reception of the triangular cardi- 

 nal tooth of the other valve ; anterior one small, and connected with the 

 anterior lateral tooth ; posterior one larger, oblique, and divided longi- 

 tudinally by a deep slit for the reception of the lamina in the corres- 

 ponding pit of the other valve. Ligament external (?) Muscular im- 

 pressions shallow. Pallial line simple (?) Surface without radiating 

 markings. 



The typical and only known species of this genus has very much the 

 external aspect of a Crassatella, from which, however, it is widely re- 

 moved, generically, by its hinge characters, though probably belonging 

 to the same family. Its muscular impressions are very faintly marked, 

 as is also the case with its pallial line, which I have not been able to 

 trace along its entire length, though enough of it can be seen to leave 

 little, if any, room for doubting that it is simple. Its lateral teeth are 

 elongated parallel to the cardinal margin, the posterior being rather 

 remote from the cardinal, while the anterior one in the right valve con- 

 nects at its posterior end with the small anterior cardinal, and in the 

 left extends back to the small pit for the reception of this anterior cardi- 

 nal tooth of the right valve, just in front of the larger, trigonal cardinal 

 tooth of the left valve. The anterior lateral tooth of the right valve seems 

 to fit under that of the left, and the posterior one of the right over that 

 of the left, though I am not positively sure of this. The cardinal teeth 

 are very peculiar, those of the right being rather widely diverging, with 

 the very oblique posterior and larger one so deeply divided longitudi- 

 nally by a very narrow slit, that it may possibly sometimes rather pre- 

 sent the character of two teeth, though the posterior half seems scarcely 

 prominent enough to be considered a distinct tooth. The principal or 

 anterior trigonal cardinal tooth of the left valve, which fits between the 

 two diverging ones of the right, is sometimes so deeply emarginate be- 



