lO 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- 

 maeroids, 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, Cestracion 

 (Fig. 91), is known to have been represented early in the 

 Mesozoic ; the Australian Lung-fish, Ceratodns (Fig. 127), 

 dates back to Liassic times;* the Frilled Shark, Chlamy- 

 doselacJie (Fig. 92), though not of a palaeozoic genus, as 

 formerly supposed (Cope), must at least be regarded as 

 closely akin to the Sharks of the Silurian. 



* Cf., however, Smith Woodward, The Fossil Fishes of the Haivkesbury 

 Series at Gosford. Memoirs of the Geol. Surv. of N. S. W. Pal. No. 4, 1890. 



