and the Elements in which they live. 389 
known to reach the mouths of rivers emptying into the sea. And 
though the families of Sharks and Skates are chiefly marine, num- 
bers 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 re- 
gions. Among Cyclostomes, the lowest type Branchiostoma is ma- 
rine, Petrostoma proper being both marine and fluviatile, the higher 
type of Ammoceetes (for we must consider Ammoceetes as higher, 
inasmuch as the division of the lips indicates a tendency towards 
a formation of a distinct upper and lower jaw), is exclusively flu- 
viatile. The Goniodonts which from their affinities to Stur- 
geons rank higher than the Siluride, are exclusively fluviatile, 
whilst there are some marine types among the latter. Among 
Percoids we find in fresh water a larger number of those in which 
the two dorsals are distinct, acharacter making them eminently 
Superior to the forms with undivided fins. or the same rea- 
son we should consider the Sparoids inferior to the Percoids, 
their dorsals being not only generally undivided, but even cov- 
ered with scales. Among the Eels, those destitute of all ffs 
are exclusively marine, those without pectorals also exclusively 
marine, and we may fairly consider the fresh water Eels as the 
higher type of the family on this ground. If there is any natural 
connection, as I have attempted elsewhere to show that there 
1s, between Scombroids and Scomberesoces, and Esoces proper, 
it becomes plain at once that the latter are the higher from the 
abdominal position of their ventrals, and they are a fluviatile fam- 
ily. Even taking the Cycloids as a whole, we find among them 
the lower families of Thoracici and Jugulares, as the families of 
Scombrides, chiefly marine, whilst the family of Salmo- 
hide, and Cyprinide are chiefly fluviatile. Among the Gadoids 
we have those with many vertical fins, as the true Cod, marine, 
while those in which the dorsals and anals are reduced such as 
the genus Lota are fluviatile. Even among the Salmonide in the 
widest extension which this family had formerly, we find the 
Scopelidae with the inferior structure of their jaws chiefly marine, 
while the Coracini and true Salmonide are chiefly fluviatile. 
Everywhere, in fact, in each minor group, the fluviatile represen- 
tatives show characters indicating their superiority over their ma- 
tine representatives. Whatever exceptions might be found to 
this law, which in the outset appears so general, I have no doubt 
will lead at some future time to the discovery of some other prin- 
ciple as yet unknown. 4 ee 
The class of Reptiles is one of the most interesting in the point 
of view under consideration, and each of their types exemplifies 
In itself the law of the intimate connection between animal types 
and the media in which they live in the most striking manner, 
ch as here the gradation, which might be inferred from 
