488 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



with parallel lateral margins. Lower valve with moderate internal con- 

 cavity, and having the appearance of a little gutter, or elongated trough ; 

 beak usually nearly straight, rather obtusely pointed, and more 

 or less distorted by the scar of attachment ; ligament area of moderate 

 size, strongly striated transversely, and provided with a large, deep 

 longitudinal furrow ; surface apparently only with moderately distinct 

 marks of growth. Upper almost nearly flat externally, but nearly as 

 concave as the other within ; beak usually a little truncated ; ligament 

 area marked with strong transverse striae, and having its. mesial ridge 

 very prominent, and occupying as much as one-third its breadth; sur- 

 face as in the other valve, or perhaps a little smoother. 



Length of adult examples about eighteen inches ; breadth of same 

 about 2.50 to 3 inches. 



Although not a very uncommon species, I have seen no entire speci- 

 mens of this remarkable shell. It will be readily known by its unusually 

 narrow, elongated, and generally straight form. The shell is usually 

 found broken into several pieces, but casts of the internal cavity are 

 not unfrequently met with entire. One of these now before me is nearly 

 one foot in length and only 2 inches in breadth. It often had a curious 

 habit of growing in groups of three shells, attached to each other by 

 the backs of their beaks. I have seen large numbers of them closely 

 arranged, or nearly in contact with each other, at Coalville, all with 

 their beaks downward, or at right angles to the planes of the sandstone 

 strata. When found where it has grown isolated, the shell is sometimes 

 arched to one side. 



Locality and position. — This species ranges through nearly the whole 

 thicknessof the Cretaceous sandstones near Coalville, Utah, and is also 

 found in the Cretaceous coal-bearing sandstones at Bear River City, 

 Wyoming, as well as in a sandstone ridge of same age on Union Pacific 

 Eailroad, a few miles east of the latter locality. 



OsTREA ANOMioiDES, Meek. 



Shell rather small, very thin, depressed-plano-convex, and without 

 any visible scar of attachment, varying from ovate to circular ; rounded 

 or sometimes a little straightened on the hinge margin ; beaks scarcely 

 projecting beyond the outline of the cardinal margin. Lower valve 

 very shallow ; cartilage pit unusually small, shallow, and short. Upper 

 valve almost perfectly flat ; cartilage attachment even shorter than that 

 of the other valve, and slightly convex on its inner margin. Muscular 

 scars unknown ; surface of both valves with small regular concentric 

 wrinkles most distinctly marked on the central region. 



Greatest diameter of one of the largest oval specimens, 1.70 inches; 

 breadth, L40 inches; convexity, 0.23 inch. 



This species is remarkable for the thinness of the shell, the slight con- 

 cavity of the under valve, and the flatness of the upper, as well as for 

 its rounded or slightly straightened cardinal margin, and the absence 

 of any scar of attachment, or of any traces of muscular impressions 

 within. These external characters, and the regular small concentric 

 wrinkles, give the exterior of the lower valve of circular specimens some- 

 what the appearance of a Lucina or Dosinia ; while in other individuals 

 it looks more like an Anomia or Placuna. 



Locality and ^position. — Missouri Eiver, below Gallatin City, Montana. 

 Cretaceous. 



