UNDOUHTED TEUTIAIIY SPECIES. 185 



move 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 3Ii/a 

 tdlinokles. 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 of 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.— ThQ typical specimens of this species were 

 brought by Colonel Simpson from a whitish limestone, containing great 

 numbers of Goniobasis Simpsoni, at Ham's Fork, near Fort Bridger, Wyom- 

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

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



