+ 146 TELEOSTOMES 
These distinguishing characters were : — 
I. Acontractile 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 Butrinus (p. 258) 
were discovered by Boas: the Teleost Chetrocentrus 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 beena 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- nm 
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, 
