236 On the Evolution of the Vertebrata, [ March, 
IV. Tue LINE OF THE PISCEs. 
The fishes form various series and subseries, and the tracing 
of all of them is not yet practicable owing to the deficiency in our 
knowledge of the earliest or ancestral forms. Thus the origins of 
the four subclasses, Holocephali, Dipnoi, Elasmobranchii and 
Hyopomata, are lost in the obscurity of the early Palzeozoic ages. 
= A comparison of the four subclasses just named shows that 
they are related in pairs. The Holocephali and Dipnoi have no 
distinct suspensory segment for the lower jaw, while the Elasmo- 
branchii and Hyopomata have such a separate element. The lat- 
ter therefore present one step in the direction of complication be- 
yond the former, but whether the one type is ‘descended from the 
other, or whether both came from a common ancestor or not, is 
unknown. If one type be derived from the other it is not certain 
which is ancestor, and whether the process has been one of ad- 
vance or retrogression. The fauna of the Permian epoch throws 
some light on the relations of these subclasses in other respects. 
The order of the Ichthyotomi,' while belonging technically to 
the Elasmobranchi, presents characters of both the Dipnoi and 
the Hyopomata. It is so near to the Dipnoi in the characters of 
the skull that nothing save the presence of a free suspensor of 
the lower jaw prevents its entering that subclass. It indicates 
that the one of these divisions is descended from the other, or 
both from a common division which may well be the group Ich- 
thyotomi itself. In case the Elasmobranchi have descended from 
the Ichthyotomi, they have undergone degeneracy, as the Ichthy- 
otomi have a higher degree of ossification and differentiation of 
the bones of the skull. If they descended from a purely carti- 
laginous type of Dipnoi, they have advanced, in the addition of 
the free hyomandibular. If the Dipnoi have descended from 
either division, they have retrograded, in the loss of the free hyo- 
- mandibular. As regards the Hyopomata, we have a clear advance 
` over the other subclasses in the presence of the maxillary arch 
_and the opercular apparatus. 
_ Too little is known of the history of the subclasses, excepting 
_ the Hyopomata, for us to be able to say much of the direction of 
~ the descent of their contained orders. On the sharks some light 
is shed by the discovery of the genus Chlamydoselachus Gar- 
See Palzeontological Bulletin No. 38, E. D, Cope, 1884, p. 572, on the genus 
