CHORDATA — VERTEBRATA — FISHES 345 



The Teleostomi are subdivided into the following orders, — 



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a. Crossopterygii 345 



b. Chondrostei 346 



c. Holostei 347 



d. Teleostei 347 



Orders a to c, inclusive, often classed as ganoids, include few 

 living forms but were exceedingly numerous during Paleozoic 

 and Mesozoic times ; their scales are thick, usually rhombic 

 plates composed of a bone-like dentine, ganoin, giving an 

 iridescent appearance, and generally united to one another by 

 peg and socket joint, hence strong and flexible. The last 

 three orders, b-d, are sometimes classed together as actino- 

 pterygians. 



Order a, Crossopterygii. — In living forms of the fringe-finned 

 ganoids the pectoral fin is made up of a solid basal portion sup- 

 ported by internal bones and a fringe of rays (whence the name 

 from Greek crossoi, a fringe, + pterygion, a small wing) . Caudal 

 fin diphycercal. In fossil forms the pectoral fin is a true archi- 

 pterygium, that is, it is a leaf-shaped fin consisting of an elongate, 

 segmented central axis bearing two opposite and more or less 

 symmetrical rows of jointed rays ; central axis and rays com- 

 posed either of cartilage or bone. 



This is an ancient order, beginning in the Devonian. It 

 combines characters of the Elasmobranchii, Dipneusti and Tele- 

 ostomi, and is represented at present by but two living species, — 

 Polypterus of the Nile valley and equatorial Africa and Cala- 

 moichthys from western Africa. Holoptychius is a Devonian 

 form widely spread in North America and Europe. In this 

 genus no ossification has been noted in the sheath of the noto- 

 chord to form vertebrae. In Eusthenopteron such ossification 

 has begun in the anterior portion (Fig. 147). 



The three remaining orders of the Teleostomi have no basal 

 fleshy lobe to the paired fins. 



