136 ARTHRODIRAN LUNG-FISHES 
plates. Among their forms appear to have been those 
whos shape was apparently sub-cylindrical, adapted for 
swift swimming; others (JZy/ostoma) 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 Dzplognathus, 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 Chimeeroid. 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. : 
