FOSSIL VERTEBRATES — FISHES 411 



to the Petromyzonts {Marsupiobra?ichii*) , from the earliest verte- 

 brates. Palaeontology, however, has lately been giving rich con- 

 tributions to this disputed problem, and there can at present be 

 little doubt that the conditions in fossil fishes have demon- 

 strated that in most ancient times Dipnoan and Teleostome were 

 closely approximated. Although even in the earliest fossils 

 they may be distinguished (e. g., by the arrangement of the 

 head-roofing derm bones), yet, as Smith-Woodward has noted, 

 forms occur too clearly transitional to indicate anything less 

 than genetic kinship. The Crossopterygian, whose ancient 

 structure is well known, may well have been derived from an 

 ancestor common to the Ctenodont (Dipnoan) and the 

 Holoptychian ; so that the gradual nearing of the Teleostome 

 stem to that more fixed, of the Dipnoan, is a strong suggestion 

 of its derivation. The later descent of the Ganoids from an 

 ancestor closely akin to, if not identical with the Crossopter- 

 ygian, is usually conceded. Teleosts, first occurring in the 

 Cretaceous, are by evidence of fossils the almost undoubted 

 survivors of an extensive group of transitional Mesozoic 

 Ganoids. But whether all Teleosts are to be deduced from a 

 single ganoidean phylum can at present hardly be established. 

 Thus catfishes, or Siluroids, appear in many structural regards 

 closely akin to the sturgeon ; but as their fossil remains are 

 lacking before the Eocene — when however, they appear to have 

 been in every way as highly evolved as in recent forms — little 

 clew has been given to their descent. 



Teleostomes may, in the present connection, be briefly char- 

 acterized in their two principal subdivisions. 



I. Crossopterygian, the more archaic group, uniting the 

 characters of shark, lungfish, and ganoid, retaining the ancient 

 cartilaginous fin bases, radials, and basals in their lobate fins ; in 

 some forms (Holoptyhchiiis) , the concrescence of the basal parts 

 of the unpaired fins passing through the same evolution as those 

 of the paired fins. Represented in the surviving PoJypterns 

 ("BicMr" of the White Nile) and in the slender Polypteroid 

 Calamoicthys (of Calabar), and in the extinct Holoptycliius, 

 Undina, Diphiriis and Coelancatlms. 



