ARTHRODIRA 537 



Some of the North American genera (e.g. Dinichthys) probably 

 attained a length of ten feet, or even, as in Titanichthys, a much 

 greater size. Some are fusiform in shape, but Mylostoma is flattened 

 and Ray-like, and, judging from the dentition, their food and 

 habits must have been equally varied. Mylostoma has tritoral 

 plates not unlike those of Neoceratodus or Chimaera. In others 

 the teeth are single, and conical or pointed ; in Titanichthys the 

 front teeth in both jaws are beak-shaped structures. It is 

 highly probable that Titanichthys, Mylostoma, and perhaps other 

 genera, are types of distinct families. 



The Arthrodira have been regarded as armoured Dipneusti, 

 a view which is mainly based on their supposed autostylism and 

 the nature of the dentition. But this autostylism has yet to 

 be verified, and, if proved, the possibility that it may be a 

 secondary feature, associated with the evolution of a peculiar 

 dentition, must not be forgotten. Much more may be said for 

 their claim to be regarded as a highly specialised race of primitive 

 Teleostomi. Besides a well-developed lower jaw, bones compar- 

 able to the elements of a secondary upper jaw are known, and 

 in a general way the disposition of the cranial roofing bones, 

 and the arrangement of the endoskeletal elements of the pelvic 

 fins, tend to conform to the normal Teleostome type. In fact, 

 Dr. Traquair has expressed the opinion that the Arthrodira are 

 Teleostomi and Actinopterygii. 1 



1 Eept. Erit. Assoc. 1900, p. 779. 



