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 ( Raia ) and Eels, {Murcena,) or are of such a for- 

 midable character as to render any security against the injuries of their class un- 

 necessary ; such are the SqualUdce or sharks, which may be said to be predomi- 

 nant among the fishes of the ocean, and the Siluridce 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. 



