138 



HISTORICAL GEOLOGY 



the fore part of the body, but none had true paired fins like the 

 Fishes. The vertebral column was of cartilage (gristle) . The eyes 

 were close together near the top of the head. They did not possess 

 true jaws in the Vertebrate sense of that term, but rather the simple 



jaw-like portions 

 moved over each 

 other laterally 

 as in many Ar- 

 thropods (e.g. 

 Beetles). 



Ostracoderms 

 reached the 

 zenith of their 

 development in 

 the Devonian, 

 and, so far as 

 known, they be- 

 came extinct 

 during the same 

 period. 



Fishes. — Be- 

 cause of the pro- 

 fusion of Fishes, 

 the Devonian is 

 often called the 

 "Age of Fishes." 

 Their abundance, 

 together with 

 bearing upon the evolution of the Verte- 

 considerable attention be devoted to the 



Fig. 80 

 Devonian Ostracoderms: a, Pterichthys testudinarius, 

 restored (Dean after Woodward); b, Tremataspis, 

 restored (after Patten). 



their importance as 

 brates, requires that 

 Fishes here. In all of our discussion of the geological history of 

 Fishes, the following important groups only will be recognized: 

 (1) Selachians ("Cartilage "-fishes), now uncommon, but e.g. 

 Sharks; (2) Dipnoans (" lung "-fishes), now rare, but e.g. Ceratodus 

 of Australian rivers; (3) Arthrodirans (e.g. Fig. 82b), now wholly 

 extinct; (4) Ganoids ("lustre "-fishes), now uncommon, but e.g. 

 Gar-pike and Sturgeon; and (5) Teleosts ("perfect bone "-fishes) , 

 now the most abundant of all Fishes, e.g. Trout, Salmon, Cod. 



Selachians are the simplest of all true Fishes, and the}^ comprise 

 the oldest group of living Fishes, dating back at least to the 



