JUKASSIC FOSSILS. 369 



arcuate, convex behind the beaks, and slightly contracted and a little con- 

 cave just in front; anterior end much narrower than the posterior, and the 

 longest point below the middle of the height. Lunule, as seen on the 

 separated valves, very narrow- lanceolate. Surface of the shell smooth, or 

 with obsolete lines of growth forming obscure concentric indulations. Fre- 

 quently a very faint depression, or sulcus, passes from in front of the beaks 

 to the antero-basal margin. The hinge line is marked by a rather strong, 

 curved cardinal tooth in each valve, with corresponding cavity or pit situ- 

 ated beneath the beaks ; the left valve having the tooth anterior to the pit, 

 and reversed in the right valve ; a small posterior lateral tooth appears to 

 exist in the left valve, and may, possibly, in the right, but it has not been 

 noticed. Muscular impressions small ; pallial line with a broad, shallow 

 sinus ; interior margin of the shell smooth and thickened. 



We have supposed this to be the shell described under the above name 

 by Messrs. Meek and Hayden, as it agrees with their description and figure 

 in its external characters. It appears, however, that they had not seen the 

 internal features. In quite a number of specimens in the collection before 

 us the internal features, as given above, are more or less clearly seen, 

 mostly by the removal of the external shell, leaving the impression of parts 

 of the hinge, and on some of the casts the pallial sinus is clearly marked. 

 This latter feature is one that does not belong to the genus Tancredia, and 

 it is with considerable hesitation that we have left it under that name ; still, 

 as we know of no other established genus to which it can be properly 

 referred, we have preferred this rather than to propose a new division on 

 insufficient grounds. The following species appears to have all the charac- 

 ters of Tancredia, except, perhaps, the pallial sinus, which has not been 

 detected on any but the present form. They are, therefore, more nearly 

 true Tancredia. 



Formation and locality. — In shaly, arenaceous limestone of Jurassic age, 

 at an elevation of 350 feet above the red beds referred to the Triassic, east 

 of the Belle Fourche River, near Bear Lodge Butte, Black Hills. 

 24 B H 



