ELASMOBEANCHI. 51 



little posterior to the middle of the length of the caudal series of vertebrae. 

 They are all depressed at the base and triangular in section beyond, and 

 have an acute ventral edge. The lateral edges are finely and rather 

 remotely serrate, the serration being obsolete on the smallest or anterior 

 spine. In all, the infero-lateral faces of the spine form a shallow groove, 

 like that of a bayonet. These spines are very different from those of the 

 Trygons of the American and European seas, where they are depressed, 

 oval in section, and have the teeth on each side much more closely placed. 



The teeth of this species are small. Viewed from below, those of the 

 upper jaw form a very few series of triangles, with their bases approxi- 

 mated and their acute apices directed backwards. Their bases are of differ- 

 ent form, and are expanded and probably bifurcate, as several sections or 

 anterior views of teeth are preserved, which exhibit two divergent roots and 

 a flat summit. The functional surfaces of the triangular crowns are flat. 



The greater number of the segments of the fin-rays are shown by the 

 sections to have been hollow cylinders, with a fibrous axis 



Measurements. 



M. 



Total length (entire) 515 



Total width at middle of abdomen 230 



Length of head (without muzzle) to scapular arch above 100 



Length of abdomen to pehdo arch 064 



Length of tail 351 



Length to origin of spine ICO 



Length of spine 040 



Depth of spine 003 



Width of base of muzzle .' 023 



Width between propterygia (greatest) 060 



Width between metapterygia (greatest) '. 065 



Width of pelvic arch iu front - 043 



This species is so far known to me from a single specimen. This was 

 obtained from Twin Creek, in the Bear River region of Southwestern 

 Wyoming, by Leslie A. Lee, of Bowdoin College, Maine, who very liber- 

 ally placed it at my disposal for study and description. 



Its presence in the Green River shales, adds to the evidence offered by 

 other anadromous types of fishes, in favor of the view that the Green River 

 Lake had commvinication with the ocean. 



The first information as to the existence of rays in the Green River 

 formation was furnished by Professor Marsh, who obtained a specimen from 



