INTERRELATIONSHIPS OF TELEOSTOMES 



147 



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 

 Holoptychian (Fig. 153) ; so that the gradual nearing of the 

 Teleostome stem to that more fixed, of the Dipnoan, is a 

 strong suggestion as to its derivation. The later descent 

 of the Ganoids from an ancestor closely akin to, if not 

 identical with the Crossopterygian, is usually conceded. 

 Teleosts first occurring in Cretaceous are by evidence of 

 fossils the almost undoubted survivors of an extensive 

 group of transitional Mesozoic Ganoids (p. 165). But 

 whether all Teleosts are to be deduced from a single 



o 



ganoidean phylum can at present hardly be established. 

 Thus catfishes, or Siluroids, appear in many structural 

 regards closely akin to the sturgeon (p. 160) ; but as their 

 fossil remains are lacking before the Eocene when, how- 

 ever, they appear to have been in every way as highly 

 evolved as in recent forms --little clue has been given to 

 their descent. 



Teleostomes may, in the present connection, be briefly 

 characterized under their two principal subdivisions. 



I. CROSSOPTERYGIAN, the more archaic group, uniting 

 characters of shark, lung-fish, and Ganoid, retaining the 

 ancient cartilaginous fin bases, radials, and basals in their 

 lobate fins; in some forms (Holoptychius, Fig. 153), the 

 concrescence of the basal parts of unpaired fins passing 

 through the same evolution as those of paired fins. 

 Represented in the surviving Polypterus (" Bichir ' of 

 the White Nile, Fig. 148), and in the slender Polypteroid 

 Calamoichthys (of Calabar), and in the extinct Holoptych- 

 ius, Undina, Diplurus, and Ccelacanthus. 



II. ACTINOPTERYGIAN, the spine-finned Teleostomes. 

 Fins supported by dermal rays ; ancient fin support greatly 



