360 THE GEEAT UNKNOWN. 



feel very diffident in attempting to touch the for:^.r 

 point, especially when so great an authority has pro- 

 nounced an opinion ; still I 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 examples are by 

 no means rare of animals much anterior, geologically, to 

 the Enaliosaurs, being still extant. The very earliest 

 forms of fishes are of the Placo'id type, and it is remark- 

 able, that not only is that type still living in considerable 

 numbers, but the most gigantic examples of this class 

 belong to it, viz., the sharks and rays ; and these ex- 

 hibiting peculiarities which by no means remove them 

 far from ancient types. The genus Chimcera 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 

 represented by two species inhabiting respectively the 

 Arctic and Antarctic Oceans. 



Now, this is exactly a parallel case to what is conjec- 

 tured of the Enaliosaurs. They appear in the oolite and 

 the chalk, are not found in the tertiary strata, but re- 

 appear, rarely, in the modern seas, represented by two 

 or more species inhabiting the Northern and Southern 

 Oceans. 



Among Eeptiles, the curious family of river tortoises 

 named Trionychidce, distinguished by their long neck, and 

 a broad cartilaginous margin to 'the small back-shell, ap- 

 pears first in the wealden. No traces occur of it in any 



