64 THE GENEALOGY OF THE VEUTEBRATA. 



tracing of all of them is not yet practicable owing to the 

 deficiency in our own knowledge of the earliest or ances- 

 tral forms. Thus the origins of the four subclasses, 

 Holocephali, Dipnoi, Elasmobranchii and Teleostomi, are 

 lost in the obscurity of the early Palaeozoic 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 Elasmobranchii and Teleostomi have 

 such a separate element. The latter therefore present 

 one step in the direction of complication beyond 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 tyjie be derived from the other 

 it is not certain which is ancestor, and whether the pro- 

 cess has been one of advancement 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, 1 while belonging technically to the 

 Elasmobranchi, presents characters of both the Dipnoi 

 and the Teleostomi. 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 di- 

 vision which may well be the group Ichthyotomi itself. 

 In case the Elasmobranchi have descended from the Ich- 

 thyotomi, they have undergone degeneracy, as the Ich- 

 thyotomi have a higher degree of ossification and differ- 

 entiation of the bones of the skull. If they descended 

 from a purely cartilaginous 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 hyomandibular. 

 As regards the Teleostomi, we have a clear advance over 



i See Paleontological Bulletin No. 38, E. D. Cope, 1884, p. 572, on the genus Didymodus. 



48 



