Arthrodires 583 



widely from the primitive Elasmobranch or Teleostome, if it 

 be admitted that in the matter of dermal structures they may 

 be clearly separated from the Chima;roid. It certainly is 

 difficult to beheve that the articulation of the head of Arthro- 

 dirans could have been evolved after dermal bones had come 

 to be formed, or that a Dipnoan could become so metamorphosed 

 as to lose not only its body 

 armoring, but its pectoral appen- X^"^ ■-""'^~\tj 

 dages as well. The size of the ^^^<^'Z^j\r^^,^^^ 

 pectoral girdle is, of course, little ■ ^^ '"' 



proof that an anterior pair of f,^ mi .-^^i Muy.ys hertz^H 



fins must have existed, since this Newberry. Upper Devonian. Ohio. 



11 1 , , , . (After Newberry.) 



may well have been evolved m 



relation to the muscular supports of plastron, carapace, trunk, 

 and head. The intermovement of the dental plates, seen es- 

 pecially in Dinichthys, is a further difficulty in accepting their 

 direct descent from the Dipnoans." 



Occurrence of Arthrodires. — These fishes occur in abundance 

 from the Silurian times to the Mesozoic. In the Devonian their 

 gigantic size and thick armor gave them the leading position 

 among the hosts of the sea. Among the genera there occurred 

 "series of forms most interesting as to their evolution." It is 

 found more and more evident," says Dr. Dean ("Fishes, Living 

 and Fossil," pp. 135, 136) " that the Arthrodirans may have rep- 

 resented the dominant group in the Devonian period, as were 

 the sharks in the Carboniferous, or as are the Teleosts in modem 

 times. There were forms which, like Coccostens, had eyes 

 at the notches of the head-buckler ; others, like Macropetalichthys, 

 in which orbits were well centralized ; some, like Dinichthys 

 and Titanichthys, with the pineal foramen present; some with 

 pectoral spines (?); some with elaborately sculptured dermal 

 plates. Among their forms appear to have been those whose 

 shape was apparently subcylindrical, adapted for swift swim- 

 ming; others {Mylostoma) whose trunk was depressed to almost 

 ray -like proportions. In size they varied from that of the perch 

 to that of a basking shark. In dentition they presented the 

 widest range in variation, from the formidable shear-like jaws 

 of Dinichthys to the lip-like mandibles of Titanichthys, the 

 tearing teeth of Trachosteus, the wonderfully forked tooth- 



