10 FISHES IN GENERAL 
Fishes hold an important place in the history of back- 
boned animals: their group is the largest and most widely 
distributed: its fossil members are by far the earliest 
of known chordates; and among its living representa- 
tives are forms which are believed to closely resemble 
the ancestral vertebrate. 
The different groups of fishes appear especially favour- 
able for comparative study. Their recent forms are gen- 
erally well understood, both structurally and developmen- 
tally; while a vast number of extinct fishes has been 
preserved to serve as a check, as well as an aid, to theoret- 
ical investigation. 
The remarkable permanence of the different types of 
_ fishes seems a striking proof of how unchanging must 
_ have ever been the conditions of aquatic living. From as 
| early as the Devonian times there have been living mem- 
bers of the four sub-classes of existing fishes, —Sharks, Chi- 
mzroids, Dipnoans, and Teleostomes. Even their ancient 
sub-groups (orders and sub-orders) usually present surviving 
members; while, on the other hand, there is but a single 
group of any structural importance that has been evolved 
during the lapse of ages,—the sub-order of Bony Fishes. 
There are many instances in which even the very types of 
living fishes are known to be of remarkable antiquity: 
thus the genus of the Port Jackson Shark, Cestvacion 
(Fig. 91), is known to have been represented early in the 
Mesozoic; the Australian Lung-fish, Ceratodus (Fig. 127), 
dates back to Liassic times;* the Frilled Shark, Ch/amy- 
doselache (Fig. 92), though not of a palzozoic genus, as 
formerly supposed (Cope), must at least be regarded as 
closely akin to the Sharks of the Silurian. 
* Cf., however, Smith Woodward, Zhe Fossil Fishes of the Hawkesbury 
Series at Gosford. Memoirs of the Geol. Surv. of N. S. W. Pal. No. 4, 1890. 
