CRETACEOUS FOSSILS. 165 



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

 rower, and have the posterior margin more distinctly truncated, while the cost* 

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

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

 the inmiediate umbones, and the posterior margin is more romid or less dis- 

 tinctly truncated, while the valves become proportionally less depressed in 

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

 identical with U. pnsrm, 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 lliver beds from which the fomi 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 IMissouri 

 country where the type of U. pnscus 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 jw5/7io;/.— Fresh-water beds on Bear River, near the mouth 



of Sulphur Creek, Wyoming. 



Unio BELLiPLicATUS, Meek. 



Plate 10, figH. 4, \ a. 

 Unio beUiplieaUis, Meek (1870), Proceed. Philosopbical Society of Pbiladelpliia, XI. 

 Unio {Loxopleurus) belUpHcatus, Meek (1873), Hayden's Second Keport Geol. Survey 

 of the Territories, 294. 

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

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

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

 posterior margin obUquely-sttbtruncated, with a more or less convex out- 

 line, most prominent below, where it is obtusely subangidar 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- 



