THE GREAT UNKNOWN. 



will modestly express one or two thoughts on 

 it. 



There does not seem any a, priori reason why 

 early forms should not be perpetuated ; and ex- 

 amples are by no means rare of animals much 

 anterior, geologically, to the Enahosauvs, being 

 still extant. The very earliest forms of fishes are 

 of the Placoid type, and it is remarkable, that 

 not only is that type still living in considerable num- 

 bers, but the most gigantic examples of this class 

 belong to it,— viz, the sharks and rays ; and these 

 exhibiting peculiarities which by no means re- 

 move them far from ancient types. The genus 

 Ghimmra appears in the oolite, the wealden, and 

 the chalk; disappears (or rather is not found) in 

 any of the tertiary formations, but reappears, 

 somewhat rarely, in the modern seas. It is repre- 

 sented by two species inhabiting respectively the 

 Arctic and Antarctic Oceans. 



Now, this is exactly a parallel case to what is 

 conjectured of the Enaliosaurs. They appear in 

 the oolite and the chalk, are not found in the 

 tertiary strata, but reappear, rarely, in the 

 modern seas, represented by two or more 

 species inhabiting the Northern and Southern 

 Oceans. 



Among Reptiles, the curious family of river tor- 

 toises named Trionychuhf, distinguished by their 

 long neck, and a broad cartilaginous margin to 

 the small back-shell, appears first in the wealden. 

 No traces occur of it in any subsequent formation, 

 till the present period, when we find it represented 

 by the large and savage inhabitants of the Missis- 

 sippi, the Nile, and the Ganges. 



What is still more to the purpose is, that the 

 IiXunnodon, a vast saurian which was contem- 

 porary with the Plesiosaur and Ichthyosaur, 

 22 337 " 



