430 NON-MARINE FOSSIL MOLLUSCA. 



beds, and which is better known in Southwestern Wyoming and the 

 adjacent parts of Utah than elsewhere, is probably somewhat earlier in 

 its origin than those divisions of the group which occur in other por- 

 tions of that great western region.* Therefore the two species of Unio 

 which those strata have furnished may be properly first considered. 



One of these species, which is represented on Plate 6, is interest- 

 ing as the type of what was regarded by Mr. Meek, who described the 

 species under the name of Unio belliplicatus,} as a distinct subordi- 

 nate type of Unio, to which he subsequently gave the subgeneric name 

 of Loxopleurus. J As this species has the true hinge structure, pallial and 

 muscular markings, and ordinary external form of Unio, its assumed 

 type characteristics consist only of its peculiar style of surface plication. 

 It may well be questioned whether this feature is sufficient to base a 

 subgeneric distinction upon, especially as the bent plications seem to 

 consist essentially of a blending of small concentric folds, which are 

 developed only near the beaks, with radiating folds, both of which are 

 respectively found upon various other species of Unio. 



The other species which has been referred to as occurring in the Bear 

 Kiver Laramie beds, and with which U. belliplicatus is usually associ- 

 ated, is Unio vetustus Meek, which -is figured upon Plate 7. This 

 species has an external form somewhat similar to that of U. bellipli- 

 catus, but its surface is plain, except that a few more or less distinct 

 concentric wrinkles are usually observable upon the beaks ; and some- 

 times one or two faint radiating raised lines appear upon each side of the 

 postero-dorsal portion. Both species have all the characteristics of true 

 Unio well developed, and both have the front shorter than is usual 

 among living species of that genus which have a like transversely oval 

 outline. In the latter feature they agree with other fossil species, which 

 are presently to be noticed. 



Those strata belonging to the great Laramie group, which are some- 

 what extensively developed in the Upper Missiouri Kiver region, and 

 which have become generally known as the Judith Kiver beds, have 

 furnished a considerable number of species of Unio, besides one species 

 of Anodonta, which has already been mentioned. 



All the species of Unio noticed in this article, especially those of the 

 Laramie and fresh- water Eocene groups, may be divided into several 

 natural sections, each section representing a subordinate type of Unio, 

 which, with one doubtful exception presently to be noticed, has repre- 



* This suggestion that the Bear River beds are older than the other portions of the 

 Lararaio Group is not made with entire confidence. It is based wholly upon the greater 

 dissimilarity that appears between the fauna of the former than that of the latter 

 when both are compared with living mollusca. This dissimilarity may, of course, be 

 due toother causes than the supposed greater length of time since the existence of the 

 Bear River fauna than has passed since that of the other portions of the Laramie 

 group was living. 



tGeol. Sur. 40th Parallel, Vol. iv, p. 165, pi. xvi, fig. 4. 



t See U. S. Geol. Sur. Terr., Vol. ix, p. 515. 



$ Geol. Sur. 40th Parallel, Vol. iv, p. 164, pi. XVI, fig. 5. 



