CRETACEOUS FOSSILS. 165 



Young specimens of this species seem to be generally proportionally nar- 

 rower, and have the posteriormargin more distinctly tmncated, while thecostse 

 or concentric ridges of the beaks cover proportionally more of the umbonal 

 region. In large examples, these markings become obsolete, excepting on 

 the immediate umbones, and the posterior margin is more round or less dis- 

 tinctly truncated, while the valves become proportionally less depressed in 

 general outline. I have long suspected tliat this shell may possibly be 

 identical with U. priscus, M. & H., as the specimens seem to be very much 

 alike. Still, as we only know the latter from very imperfect specimens, 

 while the Bear River beds from which the form here under consideration 

 was obtained, seem to be very local, and, so far as yet known, to contain a 

 peculiar fauna, almost without exception unknown in the Upper Missouri 

 country where the type of U. priscus was discovered, it is perhaps better 

 to keep these two proposed species separate until we can have an oppor- 

 tunity to compare better series of specimens from the two distantly sepa- 

 rated localities. 



Locality and position. — Fresh-water beds on Bear River, near the mouth 

 of Sulphur Creek, Wyoming. 



Unto bellipltcatus, Meek. 



Plate 16, figs. 4, 4 a. 



TJnio bellipUcatus, Meek (1870), Proceed. Philosophical Society of Philadelphia, XL 

 Unio [Loxopleurus) bellipUcatus, Meek (1873), Hayden's Second Keport Geol. Survey 

 of the Territories, 294. 



Shell attaining a medium size, transversely subovate, the widest part 

 being a little in advance of the middle, moderately convex, generally less 

 than twice as long' as high ; anterior margin short, rather regularly rounded; 

 posterior margin obliquely- subtruncated, with a more or less convex out- 

 line, most prominent below, where it is obtusely subangular or abruptly 

 rounded into the base ; dorsal outline nearly straight, or more or less 

 arched; base broad-semiovate, being most prominent in advance of the 

 middle, in large specimens generally a little sinuous posteriorly; beaks 

 much depressed, or scarcely rising above the cardinal margin, oblique, and 

 placed near the anterior end, not eroded ; hinge moderately long, with car- 



