nvhtte.] LAEAMIE FOSSILS OF YAMPA VALLEY. 209 



described and illustrated in vol. ix of the United States Geological Sur- 

 vey of the Territories. It is quite abundant at some places in the valleys 

 of the Yampa and Bitter Creek, in the upper portion of the Laramie Group ; 

 but although the genus is so well represented east of the mountains in Col- 

 orado, this species is not known there. I have at present no doubt of the 

 identity of these Northwestern Colorado #nd Southern Wyoming forms 

 with the typical forms of the species. There is a possibility that this 

 species is also identical with G. cytheriformis Meek & Hayden, originally 

 obtained from the same group and region; but as the differences between 

 the two forms there, as represented by Meek, are repeated in the valley 

 of Bitter Creek, as will be shown on a subsequent page, I prefer at pres- 

 ent to regard them as separate species. G. occidentalis is "a somewhat 

 variable species, and the shell described by Mr. Meek in t he An. Bep. 

 U. S. Geol. and Geog. Surv. Terr, for 1872, p. 513, as G. Banhisteri, is 

 no doubt specifically identical with it. This view is confirmed by find- 

 ing forms that are not distinguishable from the type specimen of the 

 latter form among those that are not distinguishable from G. occidentalis y 

 and also with associated forms that connect the two. This subject will 

 be further discussed under the head of notes on the Laramie fossils of 

 the Bitter Creek Yalley. 



6. Gorbicula (Leptestlies) fracta Meek. 



A few imperfect but unmistakable examples of this species were found 

 at the locality in question. The species has been discussed under the 

 head of notes on the Laramie fossils of Crow Creek Valley, and it will 

 be further noticed in connection with the Laramie fossils of Bitter Creek 

 Yalley. 



7. Gorbula siibtrigonalis Meek & Hayden. 



This is one of the most widely distributed species of the Laramie 

 Group, and it has also a considerable vertical range. It was found as- 

 sociated with the two foregoing species in Yampa Yalley. Under the 

 head of notes on the Laramie fossils of Crow Creek Yalley, its charac- 

 teristics, distribution, and synonymy are briefly discussed. It is known 

 to occur at two separate horizons in the valley of Bitter Creek, and will 

 be further discussed in connection with the fossils of that district. 



8. Neritina volvilineata White. 



This species has hitherto been found only west of the Bocky Moun- 

 tains, the only other locality being near Black Buttes station in the val- 

 ley of Bitter Creek, where the species was originally discovered. The 

 horizon at both localities is practically the same and near the top of the 

 Laramie Group. Most of the specimens found in Yampa Yalley are 

 considerably larger than the types, but they are without doubt specifi- 

 cally identical. The species was first described in Powell's Beport on 

 the Geology of the Uinta Mountains, p. 131. 



9. Melania icyomingensis Meek. 



A few imperfect examples only of this species were obtained in Yampa 

 Yalley. Its horizon here is practically the same as at Black Buttes Sta- 

 tion in Bitter Creek Yalley, where it was first discovered, namely, near 

 the top of the group. Its great geographical distribution and vertical 

 range have already been referred to, and will be still further discussed 

 on subsequent pages. 



10. Yiviparus plicapressus White. 



This species, like its associate, Nerltina volvilineata, has yet been found 

 only at Black Buttes Station in Bitter Creek Yalley, and in the valley 

 14 GS 



