262 INTRODUCTION 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 

 burrowing 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 

 Fig. 210. LKPIDOSIREN. them to activity.* 



We have endeavoured, with great 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 their 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 discrimi- 

 nating eye of the naturalist and the profound generalizations 

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

 four primary groups, according to the form of their scales; 



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

 2d, 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 Herring, 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. 



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

 nearly those which are here given. * 



