Phylngeny of the Teleostomi. 



335 



tlie overlying- cleithium mu^t 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 

 ])osition. 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 docs not develop 

 a posterior process for articulation with the inner face of the 

 ojierculum, as is the case in all Teleostei. 



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

 fruLir plates in a tj'pical Crossoptevygian, Choudrostean, and 

 Teleost. A. Ehizodopsis sauroides (after Traqiiair) ; B. Rhabdo- 

 lepis macropterus (after Traquair) ; C Amiii calva. /.//., inter- 

 gular; y., gular plates; L(j., lateral gulars ; b., brancLiostegals j 

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



In the Palceoniscidse 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 the bran- 



* Mon. Pala^ont. Soc, Palreonir^cidne, p. 21 (1877j. 



