Subclass Dipneusti, or Lung-fishes 623 



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

 Holostean Ganoids (Lepisostens, Ainia, etc.) should be sepa- 

 rated from the CJiondrostci and referred to the Tclcostci, 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, Psc pit urns and PolyoJoii, 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 CJiondrostci h:\xe sprung directly, through 

 ancestors of the Lysoptcri and Sclacliostoini, 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 CJiondrostci 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 caUed Crossopterygii and Piacodcrnis. 



These conclusions are summarized by i\Ir. Regan as follows: 



(i) The CJiondrostci are the most generalized Tclcostoini. 



(2) The Crossopterygii dift'er from them 



(a) in the lobate pectoral fin; 



(b) in the larger paired gular plates. 



(3) The Placodcrmi (Coccosteidcc, Astcrolcpidcr, CcpJialaspi- 



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

 straci, which are Cliondropterygii. 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 



Coeeostens; 



(b) the structure of the A-entral fin in Coeeostens; 



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



lepidec. 



