GEOLOGICAL SURVEY OF THE TERRITORIES. 515 



fisti'ongly inciirvetl, while its posterior iimbonal ridge is mncli more 

 angular. Its cartilage-process is also larger and more promineut, aud 

 the pit by its side for the reception of the tooth of the other valve is 

 proportionally smaller. As the specimen of G. suhtrigonalis is evidently 

 water- worn, however, the more rounded character of its posterior um- 

 bonal ridge may be in jiart, at least, due to that cause, though the 

 other difference mentioned could hardly have been so produced. 



Along with the typical specimens of the species here described there 

 are separate right valves, apparently of this species, a little larger than 

 the corresponding" valve of the perfect specimen from which the descrip- 

 tion was drawn up. Tiiis valve shows a distinct furrow along the inner 

 side af the lower margin for the reception of the edge of the other valve, 

 which furrow doubtless also exists in the typical specimen, but cannot 

 be seen because the two valves are united. There is also another right 

 valve, from the same locality and bed, that is proportionally much 

 higher and shorter, and has its beaks almost exactly central, so as to 

 present more nearly the outline of an equilateral triangle. This may 

 belong to a distinct but allied species 5 if not, it would indicate a con- 

 siderable variation in the outline of the species described. 



Locality and position. — Point of Eocks, Wyoming. Bitter Creek series. 



GoNiOBASis '? INSCULPTA, Meek. 



Shell terete, or elongate-subconical; volutions apparently about ten, 

 convex, or sometimes flattened-con vex, increasing gradually in size; last 

 one not much enlarged, and without an angle around the middle, some- 

 times slightly concave above in large specimens ; suture well defined ; 

 aperture unknown. Surface ornamented by distinct, regular, nearly 

 straight, or slightly arched vertical costte, about sixteen of which may 

 be counted on each volution, v^hile crossing these are smaller regular, 

 deep, revolving furrows that cut each rib into five or six little transverse 

 nodes, which, from the obsolescence of the vertical costie on the lower 

 part of the last turn, become more or less continuous revolving lines on 

 that part of the shell. 



The specimens of this species yet obtained are too imperfect to aftbrd 

 the means of giving accurate measurements. eTudging, however, from 

 some of those in the collection, large adult individuals would seem to 

 have attained a length of 1.40 inches, with a breadth of body- volution of 

 near 0.50 inch. Some of these larger specimens, consisting of three or 

 four of the lower volutions, show but a very gradual decrease in breadth 

 upward; while some of those composed of the upper part of the shell 

 indicate a divergence of about 1S° for the angle of that portion of the 

 spine. 



As none of the specimens show the aperture, it is not possible to 

 determine, from those yet seen, whether this shell really belongs to the 

 fresh-water genus GoniohasiSj or to some marine genns, although the 

 species will be readily recognized by its sculpturing. Its only known 

 associates are fragments of Ostrea and AlocUola, with Corbula undifera, 

 an association that would certainly favor the conclusion that it should 

 be referred to a marine genus, in which case it would most probably Jail 

 into Bittitmi, and have to be called B. insculpta. We have, however, 

 several examples of unquestionable fresh-water shells associated with 

 marine types, in some of the rocks of this region ; while the general 

 aspect of this shell seems, as far as known, to associate it with Gonio- 

 basis. The fact, too, that nearly all the specimens yet seen are frag- 

 mentary, seems to indicate that the species did not live in the same 



