136 



AR THR ODIRAN L UNG- FISHES 



plates. Among their forms appear to have been those 

 whose shape was apparently sub-cylindrical, adapted for 

 swift swimming ; others (Mylostomd) whose trunk was 

 depressed to almost ray-like proportions. In size they 

 varied between that of a perch and that of a basking 

 shark. In dentition (Figs. 138-144) they presented the 

 widest range in variation, from the formidable shear-like 

 jaws of Dinichthys to the lip-like mandibles of Titan- 

 ichthys, the tearing teeth of Trachosteus, the wonderfully 

 forked, tooth-bearing jaw tips of Diplognathus, to the 

 Cestraciont type, Mylostoma. The latter form has hitherto 

 been known only from its dentition, but now proves to be, 

 as Newberry and Smith Woodward suggested, a typical 

 Arthrodiran. 



The puzzling characters of the Arthrodirans * do not 

 seem to be lessened with a more definite knowledge of 

 their different forms. The tendency, as already noted, 

 seems to be at present to regard the group provisionally as 

 a widely modified offshoot of the primitive Dipnoans, bas- 

 ing this view upon their general structural characters, 

 dermal plates, dentition, autostylism. But only in the 

 latter regard could they have differed more widely from 

 the primitive Elasmobranch or Teleostome, if it be ad- 

 mitted that in the matter of dermal structures they may 

 clearly be separated from the Chimaeroid. It certainly 

 is difficult to believe that the articulation of the head of 

 Arthrodirans 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 armouring 



* The writer believes that the Arthrodirans may as well be referred to the 

 sharks as to the lung-fishes; as far as existing evidence goes, they certainly 

 differed more widely from the lung-fishes than did the lung-fishes from the 

 ancient sharks. They may, perhaps, be ultimately regarded as worthy of rank 

 as a class. 



