418 PROCEEDINGS OF THE ACADEMY OF 



Amongst Opt. Raynolds' collections now before us, from the head of Wind 

 River Valley, we also recognize, from a precisely similar bed, fragments of a 

 Unio. with great numbers of a Melania, and a beautiful little Nerilella, which, 

 although completely mineralized, re'ains it original zig-zag bands of dark and 

 light colors.* The bed containing these fossils, here, like that at the Black 

 Hills, rests directly upon the Jurassic strata, and appears to dip beneath the 

 Cretaceous, while its fossils are in exactly the same state of preservation as those 

 collected at the Black Hills. As it differs at both these localities in its litho- 

 logical characters from all of the well-defined Tertiary beds of the northwest ; 

 and its fossils are not only clearly distinct from the known Tertiary forms of 

 that region, but all present a more ancient aspect, we are still inclined to think 

 it really holds a position near the lower part of the Cretaceous series of that 

 region. 



Until this can be determined, however, upon stratigraphical evidence, we do 

 not feel warranted in assigning this formation a place in the general section of 

 the Cretaceous rocks of the northwest, since it may prove to be of Tertiary, or 

 even possibly of Jurassic age. For, as all paleontologists are aware, the re- 

 mains of fresh-water mollusca cannot be relied upon in determining the age 

 of strata, excepting where they happen to be specifically identical with forms 

 known to occur elsewhere in well established horizons. This arises from the 

 fact that they are very similar in rocks of all ages in which they are known to 

 occur, or at least that they do not present peculiar distinctive features in dif- 

 ferent formations, in so marked a decree as marine shells. 



Since the first publication of a general section of Nebraska Cretaceous Rocks, 

 based upon observations made by us in 1853, while on an expedition for Prof. 

 Hall, and subsequently published by him and one of the writers, (F. B. M.), in 

 the Memoirs of the American Academy of Arts and Sciences,f our knowledge 

 of the range and thickness of these rocks, as well as of their relations tc each 

 other, and to Cretaceous beds elsewhere, has been much extended. We have 

 also been able, from the collections subsequently brought in by various expe- 

 ditions from localities scattered over wide areas, not merely to add largely to 

 the number of species previously known to be embraced in the Cretaceous 

 fauna of this region, but we have also extended our knowledge of their geo- 

 graphical distribution and vertical range. Consequently we have now the 

 means of constructing a section of these rocks that will give a more clear and 

 accurate idea of the relative importance and thickness of the subdivisions, as 

 well of their relations to each other and of each to the whole. 



Hitherto in all the sections of these rocks published, the subdivisions have 

 been designated merely by the numbers 1, 2, 3, 4 and 5. Experience has 

 taught us that inconvenience and confusion are apt to arise from the use of 

 this kind of a nomenclature, because these larger groups are constantly liable 

 to be confounded with unimportant subdivisions of local sections, to which 

 it is almost indispensably necessary to apply numbers. This being the case, 

 we propose to designate each of these formations by a distinct name, retain- 

 ing, however, opposite each name the same number formerly used for each 

 group. In selecting names, we have preferred those derived from localities 

 to such as might be suggested by the lithological or palaeontological cha- 

 racters of the different rocks ; because, however appropriate such a name as 

 "Inoceramus Bed" or " Silicious Group" may be for a formation at a given 

 locality, it will generally be found inapplicable if we attempt to trace the rock 

 over areas of any great extent. Our names have also been selected from localities 

 where the particular formation named is known to be well developed, and 

 readily recognizable. In accordance with these views, we present the follow- 

 ing section : 



* Descriptions of the latter two fossils are given in another part of this paper. 

 t Mem. Am. Acad. Arts and Sci Boston, vol v, N. S., p. 381. 



[Dec. 



