INTRODUCTION. 
XXI 
Group of the United States are in favour of the supposition that 
they are truly autostylic. As originally pointed out by Newberry l , 
the dentition of Dinichthys is most nearly paralleled by the existing 
Dipnoan Protonterus. The recently discovered triturating plates of 
Mylostoma would have been assigned to the Dipnoi or Chimaeroidei, 
if they had not fitted certain associated mandibular hones identical 
in shape with those of the Dinichthys-type : on one page, indeed, 
Newberry terms the fish a “ Placoderm” 2 , while on another it is a 
“ Dipterine Ganoid ” 3 . The bones of the cranial shield, while 
apparently homologous throughout the group, cannot be described 
by the terms that are applicable to all Teleostomi, except perhaps 
the modern Aeipenseroids; 1 at these bones are symmetrically 
disposed with respect to the median longitudinal line, and are thus 
worthy of a nomenclature. In short, the evidence in favour of the 
autostylic character of the Coccostean fishes has now accumulated 
to such an extent, that we venture to regard them as an order of 
Dipnoi, bearing the same relation to the Sirenoidei that the Acan- 
thodians seem to hold with respect to the primitive Elasmobranchs 
(Ichthyotomi), or the Actinopterygians with respect to the primitive 
Teleostomes (Crossopterygii). Eor this order the name Arthrodira 
is suggested, in allusion to the ginglymoid articulation by which the 
cranial shield is hinged upon the anterior border of the armour of 
the abdominal region in the typical and best known genera. 
TELEOSTOMI. 
It is generally admitted that the Crossopterygian Teleostomi are 
closely related to the Dipnoi, and the Devonian representatives of 
this order tend in some degree to lessen the hiatus between the 
two great subclasses. Since, however, all the early Crossopterygii 
hitherto discovered conform to the normal Teleostome type in the 
arrangement of the bones of the cranial shield, it seems probable 
that the two groups had already diverged before the development 
of membrane-bones commenced. 
The most interesting feature of the Crossopterygii consists in the 
mode of specialization of their fins; and this, as pointed out by 
Cope, affords a satisfactory basis for the definition of the suborders. 
In all the known Palaeozoic and Mesozoic members of the order the 
1 J. S. Newberry, Eep. Geol. Surv. Ohio, vol. ii. pt. ii. (1875), p. 6. 
2 J. S. Newberry, ‘ Palasoz. Fishes N. America ’ (1889), p. 122. 
3 Ibid, p. 161. 
