46 
DAVIS : FOSSIL FISH REMAINS. 
number of individuals may have been as large as during any 
succeeding age, the diversity of form, which for convenience of 
reference we designate by the terms genera and species, has 
greatly increased ; and at the present time the repeated divergence 
of the original forms through succeeding ages has produced an 
almost endless variety, infinitely greater than at any previous 
portion of the history of the world. It is more than probable 
that this great variety of forms has been produced by repeated 
slight differentiations from the parent stock, which, rendering the 
offspring better capable of adaptation to an altered environment, 
or presenting some feature more readily transmitted has 
gradually modified the species until all trace of relationship with 
its first parents is lost. It is for this reason, amongst others, 
that the study of fossil forms is essential to a thorough knowledge 
of existing fishes ; the characters found in different families of the 
latter, now quite distinct, were originally combined or blended in 
the earlier fishes, so that the fossil fishes form links in an almost 
endless chain, which renders their study of intense interest to the 
modern biologist. The imperfections in this developmental chain 
are great ; but every new addition to the knowledge of fossil 
ichthyology, derived from new or well preserved specimens, serves 
to throw additional light on the subject and to render a complete 
elucidation of the relationships of living species more possible. 
Until a comparatively modern epoch the group of fishes with 
a bony skeleton, the Teleostei, which form by far the largest 
portion of existing fishes, is not represented in a fossil state, their 
first appearance being during the deposition of the chalk. The 
Plagiostomata or sharks have been represented from the earliest 
times to the present. Traces of this group are first observed in 
the Upper Silurian rocks of Ludlow^ and they have continued to 
exist in greater or less profusion through every succeeding 
geological epoch. In the Carboniferous series of rocks the number 
of species already discovered is very large, and comprises a con- 
