VERTEBKATE ZOOLOGY. 457 



less was originally characterized by the development of lat- 

 eral fins. The higher forms of Vertebrates manifest succes- 

 sive deviations from this primitive type. 



In the Selachians, the lateral fins are low down on the 

 sides or inferior, and the plane of their surfaces is nearly 

 horizontal. 



In the Ganoids, they evince more or less of a tendency to 

 change the plane from horizontal to oblique less in the 

 Chondroganoids ; most in the Amiids. 



In the Teleosts, pectoral fins are advanced still higher up 

 on the sides, becoming, on the whole, more and more elevated 

 as the forms diverge from the Ganoids, and become special- 

 ized as thoracic and jugular Teleocephali. 



The endoskeletal bases of the limbs are supposed to have 

 originated from ingrowth of the pair-finned skeleton at def- 

 inite areas, and the composite structures evidenced in the 

 pectoral, and probably pelvic girdles, have apparently re- 

 sulted from ingrowths at different stages. If, for example, 

 we compare the shoulder girdle of the Selachians with that 

 of the Ganoids and the Teleosts, we find that the first is 

 quite simple, and it evidently represents only the inner ele- 

 ments of the girdle of the last two types. If the sturgeons 

 are considered, it becomes plain that exostosis, or develop- 

 ment of bone externally, has first supervened, and that this 

 has subsequently penetrated inwards, and become closely 

 identified with the inner or coracoid elements. (See Gill, 

 "Arrangement of Families of Fishes," p. 9, 1872). 



The process of differentiation between fins and the limbs 

 of Terrestrial Vertebrates is best explicable by reference to 

 the Polypterids. In those fishes, the pectoral fin is support- 

 ed by two elongated bones connected with a basal cartilage. 

 This basal cartilage apparently represents the humerus, and 

 the succeeding bones the radius and ulna of the land Verte- 

 brates; while in the specialized fishes, first the humeral rudi- 

 ment disappears, and then the radial and ulnar, leaving, in 

 most of the Teleosts, only the several parallel bones at the 

 base of the fin, which are, apparently, homologous with the 

 metacarpal, the carpal probably having been developed in 

 the undifferentiated cartilage intervening in the Polypterids 

 between the radial and ulnar elements. 



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