Subclass Dipneusti, or Lung-fishes 623 



derived from the early sharks as the Chimaeras are, and that the 

 Holostean Ganoids (Lepisosteus, Amia, etc.) should be sepa- 

 rated from the Chondrostei and referred to the Teleostei, of 

 which they are the primitive representatives. 



Mr. Regan especially calls attention to the very close 

 similarity in structure of pectoral and ventral fins in the Chon- 

 drostean Ganoids, Psephurus and Polyodon, with that of the 

 anal fin in the same fishes. From this he derives additional 

 evidence in favor of the origin of paired fins from a lateral fold. 

 In his view, the Chondrostei have sprung directly, through 

 ancestors of the Lysopteri and Selachostomi, from pleuroptery- 

 gian sharks (Cladoselache) of the Lower Silurian, and the true 

 fishes on the one hand and the Crossopterygian-Dipneustan- 

 Placoderm series on the other are descended from these. The 

 absence of the lower jaw in fossil remains of Ostracophores 

 may be due to its cartilaginous structure. "There is no justi- 

 fication for regarding the Crossopterygii as less specialized 

 than the Chondrostei because they were the earlier dominant 

 group." 



These views are very suggestive and contain at least some 

 elements of taxonomic advance, although few naturalists of 

 to-day will regard the Chondrostean Ganoids as more primi- 

 tive than the fishes called Crossopterygii and Placoderms. 



These conclusions are summarized by Mr. Regan as follows: 



(1) The Chondrostei are the most generalized Teleostomi. 



(2) The Crossopterygii differ from them 



(a) in the lobate pectoral fin; 



(6) in the larger paired gular plates. 



(3) The Placodermi (Coccosteida, Aster olepidce, Cephalaspi- 



dcB) are a natural group, not related to the Hetero- 

 straci, which are Chondropterygii. They may probably 

 be regarded as armored primitive Crossopterygii, 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- 



lepidcz. 



