460 WILLIAM K. GREGORY 



Infraclass ACTINOPTERI 1 Cope. 



This infraclass includes all the remaining 'Ganoidei' and 

 'Teleostei,' the vast majority of living fishes. In fossil forms 

 these ray-finned types are readily distinguished from the lobe- 

 finned Crossopterygii or the Dipnoi. 



Principal Characters. (Compare Crossopterygii, p. 456). (1) The 

 paired fins are non-lobate, i.e., the endoskeletal parts (basals or 

 "axonosts" and radials or "baseosts") are greatly reduced, so 

 that the blade or free portion of the fin is formed entirely of 

 dermal rays. (2) The median fins are unlike the paired fins; 

 they exhibit dermal rays which articulate proximally with 

 baseosts and these in turn with axonosts, which primarily cor- 

 respond in number with the neural and hasmal spines; the median 

 fins are primitively bordered anteriorly by large fin fulcra (lost 

 in later and progressive forms) . These fulcra or ridge-scales are 

 " medium, spine-like or A _ shaped scales " (Bridge). (3) In the 

 most primitive forms there is only one dorsal fin, which may 

 give rise in the higher forms to two or more. (4) The 

 caudal fin is primitively heterocercal, but in later forms is 

 modified into the homocercal, diphycercal, and gephyrocercal 

 types. (See Appendix II.) (5) The spiracles are reduced or 

 obliterated. (6) The distal end of the hyomandibular gives 

 origin to the symplectic (sometimes absent). (7) The jaw 

 suspension is methyostylic 2 and the mandible progressively 

 simplifies by the reduction of splenials and surangulars. (8) 

 The gular plates are progressively reduced; concomitantly the 

 branchiostegals become more and more important. (9) The 

 scaly exoskeleton (sometimes reduced) consists either of (a) 

 rhombic bony scales covered with ganoine and articulating one 

 with the other, or (b) of cycloid or ctenoid scales with little 

 ■or no ganoine, the bony tissue lacking the Haversian canals, 

 or (c) of bony scutes and plates. (10) The ribs are hypaxonic 

 (haemapophyses) only. (11) Except in sturgeons, etc., the chon- 

 droskeleton largely ossifies, and the notochord is more or less 

 superseded by enveloping vertebrae. 



1 (duris t ray, Ttrepov, wing, fin.) 

 3 See footnote ' page 457. 



