Phylogeny of the Teleostomi. 347 



were characteristic of the Secondary period, including the 

 highly specialized Ichthyosauria and Pterosautia, which 

 declined and were replaced by a new race, the Mammalia, 

 derived also from the same generalized stock, so must we 

 conceive the primitive Teleostomi as giving rise to the 

 Crossopterygii, with their specialized offshoots the Dipneusti 

 and Placoclermi, and remaining dormant to develope later on 

 into the typical Chondrostei. There is no justification for 

 regarding the Crossopterygii as less specialized than the 

 Chondrostei because they were the earlier dominant group. 

 The non-recognition of the true position of Cephalaspls as a 

 specialized Asterolepid seems to have been due to its occur- 

 rence in the Upper Silurian ; but when we consider that, in 

 spite of the imperfect geological record, we know that types 

 so divergent as Cheirolepis, Tristichopterus, IJoloptychius, 

 Dipterus, Coccosteus, Homosteus, Pterickthys, and Cephalaspis 

 were already in being in the Lower Devonian, we may feel 

 assured that some of these, and numerous annectent forms 

 also, must have existed long before. 



Summary and Conclusions. 



The main results of the foregoing paper may be stated as 

 follows : — 



(1) The Chondrostei are the most generalized Teleostomi. 



(2) The Crossopterygii differ from them 



(a) in the lobate pectoral fin ; 



(b) in the larger paired gular plates. 



(3) The Placodermi (Coccosteidaa, Asterolepidae, Cephalas- 



pidse) are a natural group, not related to the Heter- 

 ostraci, which are Chondropterygii. They may 

 probably be regarded as armoured primitive Crosso- 

 pterygii, this view being most in accordance with 



(a) the arrangement of the cranial roof-bones in 



Coccosteus ; 



(b) the structure of the ventral fin in Coccosteus ; 



(c) the structure of the pectoral limb of the Astero- 



lepidae. 



(4) The Dipneusti probably originated from more specialized 



Crossopterygii, e. g. from the neighbourhood of the 

 Holoptychiidse. 



(5) The Teleostei differ in so many respects from the 



Chondrostei that they should rank as an order, in 

 which the Holostei are included. 



23* 



