246 



INTEODUCTION TO ZOOLOGY. 



in the Amazon. That of the Gambia {L. annectens, Fig. 210) 

 inhabits a part of the river which overflows extensive tracts. 



Such individuals as do 

 not follow the retreat- 

 ing waters, escape from 

 the scorching rays of the 

 African sun by buiTow- 

 ing in the mud, which 

 is soon baked hard 

 above them. There they 

 remain, in a torpid state, 

 until the return of 

 the rainy season again 

 awakes them to ac- 

 tivity.* 



Fig. 210.— Lepidosiken. 



We have endeavoui-ed, with gi'eat brevity, to exhibit one 

 class out of the many by which "the world of waters" is 

 peopled. But our knowledge of the recent tribes is imperfect 

 unless we add to it that of the extinct ; and, accordingly, the 

 study of the fishes found in a fossil state is a subject of high 

 philosophical research, involving as it does, the question not 

 only of what were theii' forms, but what were the conditions 

 under which they existed. To this inquiry M. Agassiz, of 

 Neufchatel, has devoted himself; and, in the vast series of 

 investigations which it required, has combined the discrimin- 

 ating eye of the naturalist and the profound generalizations 

 of the philosopher. By him all fossil fishes are arranged in 

 four primai'y groups, according to the form of their scales : — 



1st, Ganoid, with scales shining, as the Sturgeon. 

 2nd, Placoid, with scales broad-plated, as Sharks and Rays. 

 3d, Ctenoid, with scales comb-shaped, as the Perch. 

 4th, Cycloid, with scales of circular or smooth edges, as the 

 Cod and Herrmg.f 



The researches of Agassiz have led him to infer, that there 



• For details connected with its organization, vide Professor Owen's 

 Lectures, and Memoir in Trans, of Linn. Society, vol. xviii., part iii. It is 

 regarded by him as the representative of a distinct order — Protopteri — 

 occupying a position between the one containing the Sturgeon and that with 

 the Sharks and Rays. 



t These terms are all derived from the Greek ; the literal meanings being 

 nearly those which are here given. 



