140 HISTORICAL GEOLOGY 



almost like legs, and the covering of scales. Their skeletons were 

 cartilaginous. Their limb-like fins and peculiar lung-like air sacs 

 were more amphibian-like than fish-like characters and they 

 strongly suggest that the Dipnoans may have been the progenitors 

 of the Amphibians. 



Arthrodirans comprise a remarkable group of Fishes now wholly 

 extinct, but they were common in Devonian time. Fig. 82b 

 shows an example of a well-known genus (Coccosteus) from the 

 Old Red Sandstone. Note the bony armor covering the fore part 

 of the body, thus suggesting the Ostracoderms, though the paired 

 fins and true jaws supplied with teeth place them with the Fishes. 

 The backbone was of unsegmented cartilage. Other forms closely 

 related to Coccosteus were remarkable for size, some having 

 attained lengths up to 20 or 25 feet. Arthrodirans were probably 

 the most formidable denizens of the Devonian seas. 



Ganoids were the most highly organized and abundant Fishes 

 of the time. These were characterized by a covering of small 



lustrous plates or bony scales, usually 

 rhomboid and set together like tile, 

 rather than by overlapping as in typ- 

 ical modern Fishes. The skeletons were 

 of cartilage, though in later periods 

 they were more or less ossified as in 



„ ' . , , ,, the few modern representatives. Their 

 Structure of a Ganoid tooth. . , , ,, , iu.ii 



(After Agassiz.) internal tooth structure was often laby- 



rinthine (Fig. 83) or much like that of 

 Amphibians of later Paleozoic periods. A typical Devonian Ganoid 

 is shown in Fig. 82c. The so-called fringe-finned Ganoids were 

 externally rather similar to the Dipnoans, especially as regards 

 the paired, lobate, limb-like fins. Their intricate (labyrinthine) 

 tooth structure, character of the skull bones, and limb-like fins, 

 suggest strong affinities with the Amphibians of the later Paleozoic. 



Teleosts, which are the most common and typical modern 

 Fishes, were entirely absent from the Devonian. In these the 

 skeletons are completely ossified and the bod} r is nearly always 

 covered with overlapping scales. In marked contrast with the 

 Devonian Fishes, Teleosts always have non-vertebrated tail 

 fins. 



General Observations on Devonian Fishes. — (1) All were of 

 simple types. The most typical and highly organized Fishes so 



