WHITE ON PALEONTOLOGY. 601 



resembles the living Margaritana rugosa Barnes, but the undulations 

 are much smaller, more numerous, and occupy a proportionally broader 

 space upon the surface of the shell than they do in that species ; be- 

 sides which the species here described is a true Unio and not a Mar- 

 garitana. 



Position and locality. — Judith River group; — Bog Creek, a tributary of 

 the Upper Missouri River, Montana Territory. 



Anodonta propatoris (sp. nov.). — Shell elongate-subelliptical in margi- 

 nal outline ; valves moderately and somewhat uniformly convex ; beaks 

 small, slightly elevated above the cardinal border ; hinge-line long and 

 straight ; ventral border broadly convex ; front regularly rounded from 

 the base up to the antero-dorsal border, which is more abruptly rounded 

 to the hinge-line; postero-dorsal border oblique and slightly convex ; 

 postero- ventral border somewhat abruptly rounded from the postero- 

 dorsal to the ventral border ; cardinal border slightly thickened, but 

 entirely plain, and in all respects such as characterizes the genus Ano- 

 donta. 



Surfoce plain, or marked only by the usual lines of growth. 



Length of the largest example collected 62 millimeters ; height of 

 the same from base to beak 36 millimeters. Length of a partly grown 

 example 37 millimeters ; height 20 millimeters. 



This species is not only a true Anodonta, but in all its characters and 

 aspect it very closely resembles several living species of that genus. 

 Should perfect examples ever be obtained, it is probable that it would be 

 very difficult to say how it differs from some one of the many and closely 

 similar living forms. It is not to be denied that its separate specific 

 identity is assumed from its known antiquity rather than proved by its 

 structure and form. So far as is known to me, this is the only species 

 of true Anodonta that has ever been discovered in any of either the 

 Mesozoic or Ceuozoic strata of Western North America. 



Fosition and locality. — Judith River group ; — Dog Creek, and also 

 upon the north side of the Missouri River, near Birch Creek, Montana 

 Territory. 



PHYSIDJ3. 



BuLiNUS ATAVUS (sp. uov.). — Shell large, much elongated ; volutions 

 about seven, increasing gradually in size, moderately convex ; suture not 

 deep ; aperture comparatively small for a species of this genus, elongate- 

 subovate in outline, its length not quite equal to one-half the full length 

 of the shell; little or no callus upon the inner lip ; surface smooth or 

 marked only by very faint and very fi.ne lines of growth. Some of the 

 specimens have the appearance of having been truncated or abruptly 

 terminated at the apex. 



Length 5 centimeters ; diameter of body-volution 17 millimeters 5 

 length of aperture 24: millimeters. 



This species is remarkable for its great size and elongation, in which 



