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. 
