INDIAN CYPRINID&. 263 
55. That the external covering of animals indicates the medium in 
which they live, we know by the hairy coat of the Mammalia, the feathers 
of birds, and the scales of fishes and Amphibia. Nevertheless there are several 
families of fishes without scales, and many terrestrial Mammalia that are sup- 
plied with them instead of hair. In the naked fishes the body is defended by 
a copious oily mucous, which saves it no less effectually than scales from the 
abrasive influence of the dense medium through which they are destined to 
move, while their habits and form render the necessity for a scaly armour less es- 
sential to their safety, being capable of concealing themselves from enemies in 
sands and mud, as the Rays (Rata) and Fels, (Murena.) or are of such a for- 
midable character as to render any security against the injuries of their class un- 
necessary ; such are the Squallide or sharks, which may be said to be predomi- 
nant among the fishes of the ocean, and the Sz/uz7d@ which maintain a corres- 
ponding place in fresh-water lakes and rivers. Notwithstanding this diversity, 
M. Agassiz has turned the external covering of fishes to a happy account in the 
study of fossil species, of which the scales alone are often the only vestiges that 
remain ; thus a knowledge of eight hundred extinct_species has been attained 
that could not have been characterized by ordinary means. Although the 
researches of M. Agassiz have added largely to the science of zoology, they 
have done still more if possible for geology, by “ introducing a new element” 
into our calculations ;* no one however is more aware than M. Agassiz himself, 
of the error into which some are led in supposing the lepidermal system to 
be based on principles that are in any way at variance with the natural 
classification of animals. By that system M. Agassiz has extended our know- 
ledge of the natural characters of species, and introduced to the calculations of 
the zoologist as well as geologist, a new element by which we must be guided 
in the study of fossil fishes, rather than a principle opposed to the classification 
of these or other animals according to their structure and habits. 
* Dr. Buckland’s Brid. Treat. p. 270. 
