294 ./. K. Thacher — Median and Paired Fins. 



Plesiosaurs give too their unambiguous evidence in the same di- 

 rection. 



This limb of the latest common ancestors of the Stapedifera, this 

 typical limb of that group, has been named by Huxley, chiropter- 

 ygium* and the term will be found convenient. 



The (Jhiropterygium and the Fins of Fishes. 



The homology between the paired fins of fishes and the limbs of 

 Stapedifera has long been recognized ; but the special homologies 

 of the skeletal parts of each has been the subject of much controversy. 

 For a historical sketch of the various divergent opinions endorsed by 

 the highest authorities I must refer to the second volume of Gegen- 

 baur's Untersuchungen. 



Two pairs of limbs are found throughout the great genetic group of 

 the Gnathostomi. The chiropterygium having been established, the 

 determination of that earlier form typical for all the Gnathostomi 

 became a more pressing question. 



To an answer to this question the investigations of Gegenbaur now 

 begin to lead. 



The first part of the second volume of the Untersuchungen dis- 

 cusses the shoulder girdle, and the result is that we are now able to 

 trace clearly and surely the primordial shoulder girdle, the scapulo- 

 coracoid, throughout the Vertebrata. We are no longer in doubt as 

 to what part of the fish fin and girdle corresponds to limb and what 

 to girdle of the Stapedifera. The results of Ge'genbaur's work with 

 which we are hei'e concerned were confirmed by the later but inde- 

 pendent researches of Parker.f 



Development of the Archipterygimn, Tlieory. 



This preliminary question having been satisfactorily answered, the 

 derivation of the chiropterygium is next attempted. 



The second half of the second volume of the Untersiichungen (1865) 

 takes up the pectoral fin of fishes. Quite a number of very excellent 

 figures of numerous Elasmobranchs, Ganoids and Teleosts are given, 

 and the limb skeleton of Protopteras is discussed. The conclusions 



drawn are as follows : 



We may take as the most generalized form of limb that of the 

 Elasmobranchs, where its various parts are most plainly presented in 



* Proc. Zool. Soc. London, IS?*!, pt. i, p. 56. T. H. Huxley, On Ctratodus 

 Forsteri. 



•j- Parker, Slioulder-girdle and Sternnni. Kay Soc, IMGS. 



