1^6 TELEOSrOMES 



These distinguishing characters were : — 



I. A contractile arterial cone, containing rows of valves. 



II. An intestinal spiral valve. 



III. The interfusion (chiasma) of the optic nerve. 



It was not until these differences were shown to be of 

 little morphological importance that the two groups were 

 merged in that of Teleostomi (Owen, 1866). Thus transi- 

 tional characters in the arterial cone of Biitrinus (p. 258) 

 were discovered by Boas : the Teleost Cheirocentms was 

 found to present ganoidean intestinal characters ; and the 

 optic chiasma, as Wiedersheim * demonstrated, could no 

 longer be regarded as of taxonomic or morphological 

 value. 



The descent of the Teleostomes, like that of the other 

 groups, has long been a matter of speculation. Their affini- 

 ties with the Dipnoans are generally admitted (Giinther, 

 Gegenbaur, Haeckel, Smith Woodward). Rabl derives them 

 directly from a selachian stem, regarding the Dipnoans 

 as later evolved ganoidean forms. Beard, on the other 

 hand, even goes so far as to entirely separate the Teleo- 

 stome stem from that of the shark, lung-fish, and amphibian, 

 deriving it with a close kinship to Petromyzonts, from the 

 earliest vertebrates. Palaeontology, however, has lately 

 been giving rich contributions to this disputed problem, 

 and there can at present be little doubt that the conditions 

 in fossil fishes have demonstrated that in most ancient 

 times Dipnoan and Teleostome were closely approximated. 

 Although even in the earliest fossils they may be distin- 

 guished {e.g. by the arrangement of the head-roofing derm 

 bones, v. p. 127), yet, as Smith Woodward has noted, forms 

 occur too clearly transitional to indicate anything less 



* One form of lizard was shown to possess a chiasma of the optic nerves; 

 in its neighbouring genus the nerves were found to cross without fusion. 



