FOSSIL VERTEBRATES — FISHES 409 



anterior portion of the body was very hard and polished in 

 appearance, looking like the enamel scales of the "Ganoid" 

 fishes and covered with small tubercles that were divided by the 

 course of rather deep sensory canals. The armor plates were 

 purely dermal in character having nothing to do with the true 

 skeleton which was still made up of cartilage. One very 

 peculiar thing was the presence of a very strong joint between 

 the plates covering the head and those covering the shoulders. 

 This joint must have permitted a great degree of motion in the 

 vertical direction between the head and the body, though the 

 purpose of such a motion is not understood. The posterior 

 part of the body was covered by a thick integument entirely 

 devoid of scales such as are present in the Ostracodermi. 



The American forms of the Arthrodira were in general much 

 larger and better developed than the European ones. They 

 reached a total length of as much as ten or twelve feet and the 

 sculpture of the armor and the variety of forms presented in the 

 development of the jaws bear witness to the great variety of 

 genera developed. The arrangement of the teeth is somewhat 

 as in the Dipnoans, that is there were no separate teeth border- 

 ing the jaws but there were dental plates that were attached to 

 the edges of the jaw ; one peculiar thing about the plates is that 

 they were not fixed as in the Dipnoans but were to a greater or 

 less extent movable. In general the body form was like that 

 of Coccosteus but the modifications of the tooth plate presents 

 a very remarkable series. In Dhrictliys they are developed as 

 sharp cutting edges with a strong notch near the anterior end, 

 in Titanicthys as a simple cutting edge, in TracJiostens and 

 Diplognatlnis the edges of the plates were serrated and presented 

 the appearance of being set with isolated teeth as in the 

 ordinarv arrangement of the fish jaw. 



The last of the subclasses of the fishes is the Teleostomes. 

 This group includes the old orders Ganoids and Teleosts that 

 have been used for so long and are still commonly met with in 

 the text-books of geology. The main distinction between them 

 as seen by the geologist is the presence in the Ganoids of bony, 



