Animals and the Elements in which they Live. 219 



After this digression, let us return to our consideration of 

 the natural connection of the secondary groups of Vertebrata, 

 with the elements in which they live. 



4. General View of Vertebrata. — 1. Fishes. — Though the 

 class of fishes is entirely aquatic, we have among these 

 animals a greater number of marine types, and some which 

 are partly marine and partly fluviatile, or at periods ma- 

 rine, or at periods fluviatile ; and others which are entirely 

 fluviatile, or almost so. And though, at present, it is not 

 plain that fluviatile types, on the whole, are superior to 

 the marine types, we should not lose sight of the circum- 

 stance that the only living Sauroids which have so many 

 characters by which they may be connected with the class 

 of reptiles, and considered as the highest among fishes, 

 are entirely fluviatile ; both Lepidosteus and Palypterus 

 occur only in fresh waters ; some of the Lepidostei only are 

 known to reach the mouths of rivers emptying into the sea. 

 And though the families of Sharks and Skates are chiefly 

 marine, numbers of them, especially of those types of Skates 

 which have numerous fossil representatives during the tertiary 

 period, such as Myliobatis, are known to ascend freely the 

 rivers in tropical regions. Among Cyclostoms, the lowest 

 type Branchiostoma is marine, Petrostoma proper being both 

 marine and fluviatile, the higher type of Ammoccetes (for we 

 must consider Ammocoetes as higher, inasmuch as the divi- 

 sion of the lips indicates a tendency toward a formation of a 

 distinct upper and lower jaw), is exclusively fluviatile. The 

 Goniodonts, which from their affinities to Sturgeons rank 

 higher than the Siluridse, are exclusively fluviatile, whilst 

 there are some marine types among the latter. Among 

 Percoids, we find in fresh water a large number of those in 

 which the two dorsals are distinct, a character making them 

 eminently superior to the forms with undivided fins. For the 

 same reason, we should consider the Sparoids inferior to the 

 Percoids, their dorsals being not only generally undivided, 

 but even covered with scales. Among the Eels, those desti- 

 tute of all fins are exclusively marine, those without pec- 

 torals also exclusively marine, and we may fairly consider 

 the fresh-water Eels as the higher type of the family on this 



