304 J. K. Thacher — Median and Paired Fins. 



■X llie Dlpnoau Fin. 



In the Teleosts and Ganoids, which are in a sense intermediate 

 between the Dipnoi and Elasrnobranehii, the limb skeleton has been 

 comparatively little modified from the form in this paper set forth as 

 the typical limb skeleton of (Tnatliostomi. JSeurcely any other pro- 

 cesses than reduction of the number of rays and concrescence are con- 

 cerned. 



Now in the fin of Ceratodns the archipterygium form has certainly 

 been developed, and if tlie previous views be correct, it has been 

 developed from this series of parallel rays. 



Gilnther* has suggested one way, and Gegenbaurf another, in 

 which a row of parallel rays might transform themselves into an 

 archipterygium form. But it is possible that these fringing rays are 

 new developments. They look very much like it in Protopterus. 

 PetersJ has called attention, in this connection, to the similar struc- 

 ture of the dorsal finlets of Polyptenis, and these might throw some 

 light on the subject. For myself, I am strongly inclined to suspect 

 that the three portions of the second piece of the limb of Ceratodus^ 

 which Gilnther describes, indicate three fin-rays, and that the feather- 

 ing of one of these is a later development. The fact that Huxley 

 could find no sign of division in his specimens seems of little weight in 

 view of the complete fusion which Ave know takes place here and there 

 in median fins. 



The Limh of Stapedifera. 

 PI. LX, fig. 70 {Ichthyoaauni.s,) exhibits my view of the composi- 

 tion of the limb of air-breathing vertebrates. The dotted lines indi- 

 cate the separate rays of which it is composed. But there are other 

 ways in which it may have been derived from a series of parallel rays, 

 and I oifer this merely as the most probable interpretation so far as I 

 can now see. Fig. 71 does the same for the hind limb of Crypto- 

 branchus Japonicus. The curvature of the rays has been exhibited 

 in a marked degree quite frequently in Elasmobranch median fins. 



The Innervation of the Paired Fins. 

 I have made complete and definite observations of the innervation 

 only in a single case, namely in the pectoral fin of Mustelus canis. 

 This fin is supplied by the first 15 niyeh»nal nerves together with a 

 very small branch from the vagus. The simplest condition is seen in 

 the aborad nerves. 



* Phil. Trans., vol. clxi, Pt. ii, p. 534. f Uiiters., Hft. iii, p. 181, note. 

 X MuUer's Archiv, 1845, p. 8. 



