xx ARTHRODIRA 5\7; 
Some of the North American genera (e.g. Dinichthys) probably 
attained a length of ten feet, or even, as in 7itanichthys, a much 
greater size. Some are fusiform in shape, but J/ylostoma 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 7itanichthys the 
front teeth in both jaws are beak-shaped structures. It is 
highly probable that 7itanichthys, 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 Rept. Brit. Assoc. 1900, p. 779. 
