412 NOX-MARIXE FOSSIL MOLLUSCA. 



fossil forms as we are warranted in doing by the well-ascertained char- 

 acter of the evidence adduced. 



As to the scope of the general subject, although it embraces the three 

 categories of non-marine mollusca, namely, those of brackish-water, 

 fresh -water, and land habitat, it is still small as compared with that of 

 the great mass of that portion of the molluscan subkingdom which is 

 embraced by the marine mollusca. Not only is the diversity within the 

 three categories of molluscan forms which are herein discussed almost 

 incomparably less than that which obtains among marine mollusca, but 

 a greater proportion of the remains of formerly existing non-marine 

 than of marine mollusca, have almost certainly been destroyed as the 

 result of geological changes and other causes which will be suggested, 

 or they have failed to be preserved in an available condition for study. 

 Therefore the record furnished by the fossil non-marine mollusca is much 

 more imperfect than it is in the case of the marine mollusca. Still, the 

 scope of this subject is a very broad one, even with our present incom- 

 plete knowledge of its details, and the discussion of many interesting 

 points pertaining to it must be deferred to other occasions. 



Again, while the three non-marine categories of mollusca, especially 

 the first two, in the order in which they have been named, may be de- 

 fined from each other with a good degree of accuracy in the case of their 

 jiving representatives, yet it has not always been found easy to say 

 whether some of those fossil forms whose nearest living congeners are 

 found exclusively in perfectly fresh-water may not have lived in waters 

 which contained at least a small proportion of salt ; but this subject will 

 be further referred to on subsequent pages. Neither are we positive in 

 all cases that those species which we refer to a land habitat were really 

 land pulmonate mollusks, or that they may not have been in some cases 

 palustral pulmonates, or, possibly, gill-bearing mollusks. But generally 

 these determinations are made with much confidence, based upon the 

 kuown correlation of shell characteristics with the soft parts of the liv- 

 ing mollusks which formed them. 



The more indefinite boundary of the scope of the present article is 

 that which I have drawn between the brackish-water and marine forms. 

 The difficulty of drawing such a line arises largely from the fact that 

 some of the genera which have more or less abundant representatives 

 in brackish waters have also representatives in marine waters 5 but I 

 have regarded those strata as of brackish- water origin, which have been 

 found to contain by natural deposition forms whose living congeners 

 are found in brackish waters, even though such fossil forms are found 

 associated with those on the one hand whose living. congeners are some- 

 times, but not not always, found in marine waters ; or, on the other hand, 

 with those whose living representatives are known only in fresh waters. 

 Such a commingling of forms as is here indicated really occurs in nu- 

 merous instances, especially in the strata of the Laramie group. Thus 

 theassociatioumoneandtbesiimcsti^ 



