Fhylogeny of the Teleostomi. 



335 



the overlying cleithrum must be regarded as typical of the 

 ancestral Teleostome; whether the fusion of the posterior 

 axonosts also is corelated with this, or whether the meta- 

 pterygium was represented in the early Teleostomi by a series 

 of separate axonosts, there is no evidence to show, but the 

 structure of the pectoral in all Teleostomi is easily explicable 

 as a modification of that of Psephurus. 



In the structure of their median, as well as of the paired 

 fins, the Chondrostei are essentially primitive, and the con- 

 dition of the vertebral column also bears witness to their low 

 position. It appears to me fairly well established for both 

 living forms and for those extinct ones which undoubtedly 

 belong to this order that the hyomandibular does not develop 

 a posterior process for articulation with the inner face of the 

 operculum, as is the case in all Teleostei. 



Fig. 1. — Diagrams to show the arrangement of the hranchiostegals and 

 gular plates in a t} r pical Crossopterygian, Chondrostean, and 

 Telecst. A. Rhizodopsis sauroides (after Traquair) ; B. Bhabdu- 

 lepis macroptertts (alter Traquair) ; C. Amia calm. i.g., inter- 

 gular; y., gular plates; iff., lateral gulars ; b., hranchiostegals j 

 c.h. t cerato-hyal ; s.op., suboperculum ; mn., lower jaw. 



In the Palteoniscidse the arrangement of the plates supporting 

 the gill-membranes and extending forward between the man- 

 dibular rami, as described by Traquair *, is one from which the 

 conditions which obtain in other Teleostomi are readily deriv- 

 able. On each side there is a continuous series of obviously 

 homologous plates, the upper two or three of which are en- 

 larged as the opercular bones, those following being thebran- 



* Mon. Falaeont. Soc, Paheoniscidne, p. 21 (1877). 



