38 NON-MARINE FOSSIL MOLLUSCA. 



in connection with Anomia propatoris and Cyrena carletoni. It was 

 originally described by Meek under the name of Melampusf antiquus, 

 but it is quite certain that it does not strictly belong to that genus. It 

 is illustrated by several figures on Plate 5. 



An imperfect example of a similar and apparently congeneric form, 

 which is represented by Fig. 17 on Plate 5 was found in the same 

 neighborhood among the remains of marine Cretaceous mollusca, as 

 was also an imperfect example of Physa; both of which specimens 

 were no doubt drifted from the then adjacent shore, and sunk among 

 the shells of marine mollusks that then lived there.* 



In the Bear River Laramie beds in Southwestern Wyoming there 

 have been found two species closely related to the two that are referred to 

 Melampusf, which, from their characteristics as well as their brackish- 

 water faunal associations are referred to the Auriculidse. They have 

 considerable resemblance to Melampusf antiquus, and are possibly con- 

 generic with it; but Mr. Meek proposed for them the generic name of 

 Rhytophorus.] 



The first of these two species was discovered by Mr. Meek, and de- 

 scribed by him under the name of Rhytophorus prisons ;£ and the second 

 was described by myself under the name of R. meehii.^ Both these 

 forms are illustrated by figures on Plate 8. 



These three or four species constitute the only representatives of the 

 Auriculidse that haveyet been obtained from North American non-marine 

 strata, and they are so remotely allied with living representatives of that 

 family that we cannot regard them as holding such ancestral relation to 

 any of the living North American Auriculidae as the fossil Unionidae and 

 some other families herein discussed, evidently hold to the living rep- 

 resentatives of those families respectively. Indeed, for reasons already 

 stated, it is practically certain that the lines of descent from such of the 

 mollusca of the Larainie period as required a saline habitat were neces- 

 sarily broken at the close of that period, when the waters of that sea 

 became wholly fresh and greatly reduced in extent. 



The earliest North American strata in which remains of the Lim- 

 nseidse have been discovered are those of the Laramie Group ; if we ex- 

 cept the Jurassic form, which was described by Meek & Hayden under 

 the name of Planorbis veternus, and which is presently to be further 

 mentioned. In the Laramie Group, however, we find the family repre- 

 sented by all the principal genera that are known among the living 

 Limna3ida3, as well as the greater part of the subordinate sections of the 



*See An. Rep. U.S. Geol. Sur. Terr, for 1878., Part I, p. 25. lb. for 1877, p.307. 

 t For diagnosis of this genus see U. S. Geol. Sur. 40th Parallel, vol. iv, p. 175. 

 t U. S. Oeol. Sur. 40th Parallel, vol. iv. p. 175, pi. xvii, fig. 6. Also Simpson's Eep. 

 Great Basin, Utah, p. 3G4, pi. v, fig. 4. 

 $ See An. Eep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 82, pi. 30, fig. 8. 



