494 GEOLOGICAL SURVEY OF THE TERRITORIES. 



ratbor dein'ossed, oblique, incurved, and placed near the anterior end; 

 uuibonal slopes not prominently rounded ; surface merely showing line, 

 rather obs(nire marks of <>Towth; anterior muscular impression rather 

 strongly defined and ob!i(|uely ovate; posterior muscular impression 

 larger and obscure; i)allial line showing a deep, angular, ascending sinus; 

 posterior lateral teeth of lunge very long, linear, aud nearly or quite 

 smooth; anterior short; cardinal teeth very oblique. 



Length of a s])ecimen, a little under medium size, 1.35 inches; height, 

 0.39 inch ; convexity, 0.08 inch. 



This species is more depressed and elongated than any of those hith- 

 erto describe<l i'rom the far-western localities, excepting one or two from 

 the coal formations on Bitter Creek, Wyoming, from which it differs ia 

 having its beaks placed farther forward. It will also be readily distin- 

 guished from those shells, as well as from all of the other species of the 

 genus yet known, from any of our rocks, by having an angular, ascend- 

 ing, and comparatively deep sinus in itS4)allial line, almcst like that seen 

 in many types of the Veneridw. This character is so strongly marked 

 that it was not until I had succeeded in getting a tolerably clear idea of 

 the nature of the hinge that I could believe the shell rela-f ed to the group 

 to which I have referred it. As was pointed out by Mr. Try on, some 

 years back, the existing American species of Cyrena aud Corbicula have 

 the pallial line more or less sinuous; while in nearly all of those from 

 foreign countries it is simple. I have also ascertained tha-t nearly all 

 the extinct North xVmerican species yet known have the pallial line sin- 

 uous. The sinus, however, is usually shallow aud rounded, or obtuse, 

 in our fossil species ; that of the shell here under consideration being 

 unusually deep and angular. 



Locality and position. — Near Missouri River, below Gallatin City, Mon- 

 tana, where it occurs, associated with Tri<jouia, Inoceramus, CardiurHj 

 Ostreaj aud other marine Cretaceous fossils. 



Corbicula Cyrena (?) securis, Meek. 



Shell (as determined from internal casts) ovate-subtrigonal, moder- 

 ately convex in the central and umbonal regions, and cuneate behind ; 

 anterior end short, with its margin regularly rounded from below the 

 beaks into the base; posterior margin apparently a little truncated, with 

 a slight backward obliquity from the posterior dorsal slope to the i)os- 

 terior basal extremity, which rounds abruptly into the ventral margin; 

 dorsal outline declining rather distinctly backward from the beaks; basal 

 margin forming a semi-elliptic or semi-ovate curve; beaks prominent, 

 gibbous, located about one-third the length of the shell from the anterior 

 margin, and rather strongly incurved ; posterior umbonal slopes some- 

 what prominently rounded; muscular imi)ressions shallow. (Surface 

 aud hinge unknown.) 



Length, 1.07 inches; height, 1.47 inches; convexity, about 1.15 inches. 



The only specimen of this species yet known being merely a cast, it 

 is not possible to determine from it, with certainty, the generic charac- 

 ters of the shell. It agrees, so nearly, however, in general appearance, 

 with Corbicula Durkeei, from the estuary beds near the same locality, 

 that I was at first inclined to believe it might be that species. A care- 

 ful comparison, however, with that shell, both as represented by inter- 

 nal casts, and by specimens showing the exterior, leaves no doubt that 

 it IS at least specifically distinct. A marked character seen in all the 

 numerous specimens of G. Durlceci is the very strong inflection of the 

 margins of the valves along the posterior dorsal slope, causing a pro- 



