Ch. V.] 



IN CHRONOLOGICAL ORDER. 47 



making a systematic arrangement by reference to organic 

 remains. 



Although the bones of mammalia in the tertiary strata, and 

 those of reptiles in the secondary, afford us instruction of the 

 most interesting kind, yet the species are too few, and confined 

 to too small a number of localities, to be of great importance in 

 characterizing the minor subdivisions of geological formations. 

 Skeleton of fish are by no means frequent in a good state of 

 preservation, and the science of ichthyology must be farther 

 advanced, before we can hope to determine their specific cha- 

 racter with sufficient precision. The same may be said of 

 fossil botany, notwithstanding the great progress that has 

 recently been made in that department ; and even in regard to 

 zoophytes, which are so much more abundant in a fossil state 

 than any of the classes above enumerated, we are still greatly 

 impeded in our endeavour to classify strata by their aid, in 

 consequence of the smallness of the number of recent species 

 which have been examined in those tropical seas where they 

 occur in the greatest profusion. 



Fossil remains of testacea of chief importance. The testacea 

 are by far the most important of all classes of organic beings 

 which have left their spoils in the subaqueous deposits ; they 

 are the medals which nature has chiefly selected to record the 

 history of the former changes of the globe. There is scarcely 

 any great series of strata that does not contain some marine or 

 freshwater shells, and these fossils are often found so entire, 

 especially in the tertiary formations, that when disengaged from 

 the matrix, they have all the appearance of having been just 

 procured from the sea. Their colour, indeed, is usually want- 

 ing, but the parts whereon specific characters are founded 

 remain unimpaired ; and although the animals themselves are 

 gone, yet their form and habits can generally be inferred from 

 the shell which covered them. 



The utility of the testacea, in geological classification, is 

 greatly enhanced by the circumstance, that some forms are 

 proper to the sea, others to the land, and others to freshwater. 



