WHITE.] INTRODUCTORY REMARKS. 413 



&c., is held to indicate a brackish- water origin for such a stratum, even 

 though it holds, associated with those shells, such forms as Unio, Vivi- 

 pqrm, Goniobasis, &c., especially if such strata alternate (as is often the 

 case in the Laramie Group) with strata which contain on the one hand 

 only such fresh-water forms as Unio, ViciparuSj &c., and on the other 

 hand such saline-water forms as Ostrea, Anomia, &c. This view is also 

 confirmed by the fact that in the Laramie Group Corbicula, Corbula, and 

 Neritina are frequently found so associated with Ostrea and Anomia as 

 to plainly indicate that they all lived together. It is thus clearly shown 

 that a part of the forms discussed in this article have near allies in strata 

 of marine origin, and also many near allies now living in truly marine 

 waters, and none in fresh waters j but the commingling of these fossil 

 species of undoubted saline habitat with others which clearly indicate 

 that they lived in water of far less than marine saltness, makes it neces- 

 sary to regard the former as members of a brackish- water fauna, and, 

 therefore, as coming within the scope of this article. 



In some cases the brackish waters that, by the presence in the depos- 

 its they have left of such fossils as have been referred to, are indicated 

 as having then prevailed, were plainly those of estuaries, which indented 

 the coasts of formerly existing seas at the mouths of then existing rivers. 

 But the greater part of the species enumerated in this article, which are 

 regarded as having had a brackish-water habitat, come from strata 

 (namely, those of the Laramie Group) which bear evidence of having 

 been deposited in a great inland sea, in parts of which sea brackish 

 waters alternated with fresh, or nearly fresh- waters. The facts upon 

 which this conclusion is based have been presented in other publications, 

 and are repeated to some extent upon following pages in this article. 



Before proceeding with the enumeration of the specific forms of 

 non-marine mollusca which have been discovered in ^North American 

 strata, and the geological position which each fauna represented by 

 them respectively occupies, it is necessary to present a brief tabular 

 statement of the arrangement or order of the geological formations, 

 beginning not with the earliest known fossiliferous rocks in the geologi- 

 cal series, but with the formation which has furnished the earliest known 

 molluscan forms that come within the scope of this article, namley, Devo- 

 nian. 



This tabular view of the formations has been made with especial ref- 

 erence to those which have been recognized in Western North America, 

 because it is there that the greater part of the fossils have been collected 

 which are discussed in the following pages. 



It may not be eiftirely unnecessary to state that, although the terms 

 group and formation are somewhat variously used by different writers, 

 the strata that were formed in a period or epoch of geological time are 

 usually and properly referred to by the same name as that of the period 

 or epoch in which they were formed. Thus, the following table of geo- 

 logical time is really a table of the formations that were produced dur 

 ing that time. 



