THE DEVONIAN PERIOD 



141 



common today, did not exist in the Devonian, and even the Ganoids 

 were of primitive types. 



(2) All had cartilaginous skeletons. The vertebral column and 

 other portions of the skeleton were not ossified (i.e. changed to 

 bone) . 



(3) All had vertebrated tail fins. The vertebral column ex- 

 tended through the tail fin and gave off fin rays to support a lobe 

 above and below. Sometimes 

 this tail fin was symmetric and 

 sometimes asymmetric. The 

 asymmetric form is regarded 

 as the more primitive. Most 

 modern Fishes (Teleosts) have 

 non-vertebrated tail fins, the 

 fin rays being sent out from a 

 plate at the end of the verte- 

 bral column. 



(4) They were generalized 

 types. " Along with their dis- 

 tinctive fish-characters, they 

 combined others which connect 

 them with higher Vertebrates, 

 especially Amphibians, and still 

 others which are found in the 

 embryos of Teleosts. The most 

 important connecting charac- 

 ters . . . are: (a) An external 

 protective armor of thick bony 

 plates or scales, such as were 

 possessed by early Amphibians, 

 and by many Reptiles of the 

 present time; (b) Large conical 

 teeth channelled at the base, and of labyrinthine structure on sec- 

 tion. This structure was very marked in early Amphibians; (c) A 

 cellular air-bladder . . . capable of being used to some extent 

 as a lung; (d) In many cases paired fins which had something- 

 like jointed legs running through them; (e) The Tail fin verte- 

 brated as in Reptiles." 1 The most prominent embryonic charac- 

 ters were the cartilaginous skeleton found only in the embryonic 



1 J. Le Conte: Elements of Geology, p. 356. 



Fig. 84 

 Types of Fish tails: a, vertebrated 

 symmetric; b, vertebrated non- 

 symmetric; c, non-vertebrated sym- 

 metric. (Redrawn after Le Conte.) 



