BEPORT ON PAL2EONTOLOGICAL COLLECTIONS. 361 



MOLLUSCA. 



LAMELLIBRANCHIATA. 



Genus UNIO, Retzius. 



Unio vetustus, Meek. 



Plato 5, fig. 12, a, o. 

 Unio vetustus, Meek (July, 1860), Proceed. Acad. Nat. Sci., Philad., XII, 312. 



Shell rather thin in young, bat becoming proportionally thicker with ago, attaining 

 a medium size, transversely-ovate, moderately convex; anterior side rounded; basal and 

 dorsal margins nearly straight and parallel in the young, but the former more convex in 

 the adult; posterior side very long, more compressed, and rather narrower than the other, 

 obliquely truncated above and angular below in voting shells, but becoming more 

 rounded with age; beaks small, much depressed, located near the anterior end; surface 

 of young specimens ornamented by line, regular, concentric wrinkles, crossed on the 

 posterior umbonal slopes of each valve by two sharply-delhicd linear ridges, which 

 radiate from the beaks nearly or quite to the posterior extremity. ( hi old and medium- 

 sized specimens, these markings become nearly or quite obsolete, excepting near the 

 beaks. 



Length of a large specimen, 3.22 inches; height, 1.30 inches: convexity, about 

 0.60 inch. 



The nature of the matrix in which these specimens are imbedded, is such that it 

 was found impossible to remove it from the hinge and interior, so as to see all the 

 details of the teeth and muscular impressions; but by working it away with care from 

 the hinge, I was enabled to determine beyond doubt that it is a Unio. 



In surface-markings, young individuals of this species bear considerable resem- 

 blance to young specimens of U. priscus, Meek and Ilaydeu. from the Tertiary deposits 

 of the Upper Missouri, with which I have sometimes thought them identical. Until we 

 can have better specimens, however, of the Upper Missouri shell for comparison, it 

 will be better to keep them separate, especially as the relative geological positions of 

 the beds in which the two forms occur still remain doubtful, while the Bear River 

 beds seem to be very local in Wyoming. 



Locality and position.— Brackish- or fresh- water beds on Bear River near the mouth 

 of Sulphur Creek; latitude, 41° 12' north, longitude, 110° 52' west: probably belong- 

 ing to the latest division of the Cretaceous. 



Genus CORBULA, Bruguiere. 

 Coebuxa (Akisoehynchus) pybefoe]\iis, Meek. 



moderately thick, very 

 a nd s u br o s t rate behind ; 



