UNDOUBTED TERTIARY SPECIES. 185 



more than once and a half as long as high, moderately convex, the greatest 

 convexity being usually slightly above and in advance of the middle; ante- 

 rior end rounded; posterior end more or less regularly rounded, being 

 usually most prominent a little below the middle ; basal margin forming a 

 semi-elliptic curve; dorsal outline somewhat straightened, or slightly convex, 

 along the central region, and rounding into the anterior and posterior mar- 

 gins, but more abruptly to the former; beaks much depressed and incon- 

 spicuous, located about half-way between the middle and the anterior 

 margin; hinge equaling about two-thirds the length of the valves; lateral 

 teeth rather long and nearly straight; cardinal teeth unknown. Surface 

 smooth, excepting small striae of growth. 



Length, 1.60 inches; height, 1 inch; convexity, 0.64 inch. 



This species is mainly characterized by its regular, nearly elliptic out- 

 line, inconspicuous beaks, and smooth surface. Its shorter and more elliptic 

 form, and especially its smooth umbones, will at once distinguish it from 

 both of the Bear River species. I have sometimes suspected that this shell 

 might possibly be the same figured and described by Professor Hall, in 

 Fremont's Report, page 307, plate iii, fig. 1 (not fig. 2), under the name Mya 

 tellinoides. Still, that figure seems to represent a shell with a more atten- 

 uated posterior and more prominent beaks. At any rate, it may be regarded 

 as extremely improbable that the figure cited, if correctly drawn, repre- 

 sents a shell even nearly related to Mya; and, although not agreeing exactly 

 with any o£ the known Uniones from that region, it very probably represents 

 a species of that genus. I greatly regret not having an opportunity to com- 

 pare our western fresh-water shells with those figured in Fremont's Report, 

 the types of which are in Albany, N. Y. I saw them all many years back; 

 but, not being at that time interested in the study of such forms, I retained 

 no very distinct recollection of their specific characters, while the published 

 description is too brief to aid one much in identifying them. 



Locality and position. — The typical specimens of this species were 

 brought by Colonel Simpson from a whitish limestone, containing great 

 numbers of Goniohasis Shnpsoni, at Ham's Fork, near Fort Bridger, Wj^om- 

 ing TeiTitory. Dr. Hayden has also obtained specimens of it from near the 

 same locality. It belongs to the Bridger Group of the Tertiary series. 



