GENERAL OBSERVATIONS. 77 



In the following brief review of the faunal characteristics of the dif- 

 ferent groups represented in the collections it is well to consider how far 

 any peculiarities they present, different from those which characterize their 

 position in time, may have been occasioned by the then and there prevail- 

 ing physical conditions under which the strata were accumulated. Some of 

 those conditions were limited both in extent and duration, but others were 

 of a more general and constant character. Almost all of the fossils of these 

 collections are the remains of mollusks and other aquatic animals. It is a 

 well-known fact that the character of the material composing the bottom 

 upon which such animals live constitutes one of the most important elements 

 in their habitat; that not only species, but even genera and families, are often 

 separated from each other in the same waters by a difference in the character 

 of the material composing the bottom. It is this gradually-accumulating 

 bottom material that has constituted the strata from which we now obtain 

 the fossil remains. The prevailing material of the strata which have furnished 

 the greater part of the fossils of our collections, especially those of Mesozoic 

 and Cenozoic ages, having an aggregate thickness of not far from three and 

 a half miles, is sand. The bottom of the waters, salt, brackish, and fresh, 

 of all the periods of both those ages, was almost constantly, either wholly 

 or in very large part, composed of sand. 



Such a condition as this, continued for so long a time, necessarily pro- 

 duced a marked effect upon the faunae of those periods, giving them different 

 or modified characteristics as compared with those of the same periods respect- 

 ively in other parts of the world. 



Again, the evidence afforded by both the vertebrate and invertebrate 

 collections, and by those of the florae of all the periods named in the 

 section on a previous page, is in favor of the existence during that time of 

 a warm-temperate and uniform climate. Furthermore, the labors of the 

 field geologists have shown that there is a great degree of conformability 

 of the strata of all the subordinate groups, from those of the Carboniferous 

 to those of the Tertiary periods, inclusive ; or at least that the cases of un- 

 conformability are few and of comparatively slight degree. So great a 

 degree of uniformity in such important conditions as these having prevailed, 

 it is not strange that the groups of strata of the different periods respect- 



