WHITE ON NEW INVERTEBRATES. 713 



Transverse length of a medium-sized specimen 30 millimeters ; height 

 from base to beak 26 millimeters; thickness 20 millimeters. 



This species differs too materially from any known described species 

 to need detailed comparison. 



Position and locality. — Laramie Group Valley of Crow Creek, Colo- 

 rado, 15 miles north of its confluence with South Platte Eiver. 



Subgenus Leptesthes Meek. 



Gorbicula {Leptesthes) macropistha (sp. nov.). 



Shell small, longitudinally subelliptical or subovate, broader (higher) 

 posteriorly than anteriorly, slightly gibbous or somewhat compressed ; 

 test strong but not massive ; basal margin broadly convex, posterior 

 margin truncating the shell, and its direction being upward and a little 

 backward, and abruptly rounded to both the postero-cardinal and basal 

 margins ; postero-cardinal margin broadly convex ; antero-cardinal 

 margin nearly straight and directed obliquely downward and forward 

 to the front, which is abruptly rounded to the base; beaks depressed, 

 not well defined, and not projecting above the hinge-line, placed about 

 one-third the length of the shell from the front. Surface showing the 

 usual lines and imbrications of growth, and well-preserved examples 

 show that the former were so fine as to give an almost polished aspect 

 to the surface. Lateral teeth well developed and finely crenulate, car- 

 dinal teeth well developed, and having the usual characters of the genus; 

 pallial line somewhat distant from the margin; sinus shallow. 



Length of an average-sized example, among the typical examples of 

 the collection, 21 millimeters ; height 15 millimeters ; thickness, both 

 valves together, 10 millimeters. 



There are two or three examples in the collection, that were obtained 

 from layers separated by only a few feet, that are considerably larger 

 than the above dimensions, but these, having some other modifications 

 of form, are referred to this species with doubt. 



This shell evidently belongs to the section designated as Leptesthes by 

 Meek. Among other peculiarities of this section, internal casts of it 

 show a distinct but shallow and somewhat broad furrow, extending 

 downward and forward from the hinge-margin behind the beaks to 

 about the middle of the shell ; and the inner surface of the valves show 

 the corresponding ridge. This, in this species at least, is really not so 

 much a true ridge as it is a sudden thinning of the shell, along a nearly 

 vertical line, in its posterior half. 



The peculiar flattening of the umbonal and upper middle portions of 

 the shell, its greater width, and equal if not greater thickness behind 

 than in front, are characters by which the species may be readily 

 recognized. 



Position and locality. — Laramie Group, Crow Creek, Northern Colo- 

 rado, 15 miles above its confluence with the South Platte River. 



