472 PROF. ST. GEORGE MIVART ON THE 



the ventral azygos fin-fold. The peculiar and apparently essentially axial nature of the 

 solid supports of the ventral part of the caudal fin seems to tell against this hypo- 

 thesis; but it need not necessarily do so, since, if we may suppose these subcaudal 

 ingrowths in certain forms to have taken place very early, they would have acquired an 

 integration with the skeletal axis much surpassing that of their dorsal homotypes, with 

 a resulting diiference of development and appearance such as we find now in the caudal 

 structures here noticed. 



But the caudal fins of difierent kinds of fishes may have arisen in difierent ways in 

 different cases, and apparent superficial resemblances may be due to the action of homo- 

 plasy. The ventral portion of the caudal fin of Murcena, that of most Teleosteans, and 

 that of Elasmobranchs may have been derived independently from the primeval caudal 

 fini. 



It remains to consider the relations existing between the paired fins of fishes and the 

 limbs of higher Vertebrates, and also the development of the cheiropterygiura. 



Before entering upon detailed comparisons, it must first be determined which part 

 of the pectoral fin is preaxial and homologous with the radial side of the arm and 

 hand? 



As to the first question, I think there can be no doubt that the view entertained by 

 Cuvier and Huxley is the right one, and that the preaxial margin of the pectoral fin is 

 that margin which is turned obliquely dorsad. 



To this view Professor Gegenbaur, though at first opposed to it, has now adhered ^ 

 as the more probable ; and I marvel that Mr. Thacher still prefers the opposite one ^. 

 I marvel all the more because his own paper seems to me to be by itself capable of 

 demonstrating the truth of the view which he is disposed to reject; for it shows* (as 

 was long ago shown by Swan ^) that the more preaxial spinal nerves go to the dorsad 

 or propterygial side of the fin, while the more postaxial spinal nerves go to the ventrad 

 or metapterygial side of the fin. 



If, then, the view here advocated is correct, i. e. if the true pectoral archipterygium 

 had a skeleton like that of the first dorsal fin of Chiloscylliuin, the second dorsal of 



' Since this paper was written, Mr. Alexander Agassiz has very kindly sent me a paper of his (from the 13th 

 yol. of the ' Proceedings ' of the American Academy of Arts and Sciences) on the development of the tail in some 

 species of fishes. In this very interesting memoir the frequent absence of vertical homology between the parts 

 respectively on the ventral and dorsal sides of the upturned extremity is made very evident, the ventral portion 

 being a secondary growth in the Flounder, and in Atherina, Batradius, Lumpus, Ctenolabrus, Poronotus, 

 Lophius, and Gasterosteus. 



- Morpholog. Jahrbuch, 2nd. vol. 3rd Heft, "Zur Morphologic der Gliedmassen der Wirbelthiere," p. 396. 



^ He says, " The weight of evidence seems to me in favour of the view that the metapterygial 



edge of the fish-fin corresponds with the radial or thumb-side of the hand." 



* In a section on " The Innervation of the varied Fins in Mustelus canis " (I. c. p. 304). 



' See Swan's " Illustrations of the Comparative Anatomy of the Nervous System," 1864, p. 32, plate xi. 



