THE ORDERS OF TELEOSTOMOUS FISHES 461 



The infraclass Actinopteri exhibits an amazing variety of 

 forms grading from the shark-like cartilaginous Sturgeons and 

 Paddle-fishes to the most specialized bony fishes. Two more or 

 less continuous major groups or series are recognized, a more 

 generalized ancient series the Ganoidei (term used in the sense 

 explained on p. 444), and a more specialized modern series the 

 Teleostei. Because morphologically annectant forms are nu- 

 merous, it is difficult to draw a hard and fast taxonomic line 

 between the two groups. 



Morphological Transition from Lower to Higher Types. 1 In 

 the more generalized fossil Ganoids (Acipenseroidei) : (1) the 

 notochord is persistent (though strengthened by neural and 

 haemal arches), (2) intermuscular orepipleural bones are absent, 

 (3) infraclavicular plates are retained, (4) the scales are rhombic, 

 bony, with a heavy ganoine lacquer, often with a peg-and- 

 socket .articulation, (5) the dermal fin rays are much more 

 numerous than their endoskeletal supports, (6) large fulcra 

 strengthen the median fins anteriorly, (7) the tail is strongly 

 heterocercal (save in Belonorhynchidae) , (8) baseosts (radials) 

 persist in both pectoral and pelvic fins. But the higher or Hol- 

 ostean Ganoids (e. g. Caturus, Leptolepis, see page 464) approxi- 

 mate more and more to the Teleosts. Usually (1) the notochord 

 is surrounded or replaced by ring-like ossifications (pleuro- and 

 hypocentra) which finally (e. g. in Oligopleuridae) become 

 perfect vertebras; (2) the scales, losing the peg-and-socket articu- 

 lation, most of the ganoine, and the Haversian canals in the bone, 

 become rounded to cycloidal, and deeply overlap; (3) the infra- 

 clavicles are reduced or wanting, functionally replaced by the 

 cleithra, or "clavicles"; (4) intermuscular (epipleural and 

 epineural) bones appear, giving the muscles better control of the 

 backbone, while the development of a bony supraoccipital gives 

 the body muscles a better hold upon the head; (5) the fins gradu- 

 ally lose the fulcra and the dermal rays and become closely cor- 

 related by reduction with their endoskeletal supports; (6) in the 

 tail fin an upturning and abbreviation of the caudal axis causes 

 an approach toward true homocercy (Appendix II) ; (7) baseosts 

 disappear from the pelvic and are reduced in the pectoral fins; 



» See Plate XXX. 



