284 



./ K. Thacher — Median and Faired Fins. 



Figure 4, PI. XLIX, shows the relation between the neural arches 

 and the segments of the lateral muscles. There are two arches to 

 one seo-ment. In fio-ure 1, then, there must have been lU segments, 

 which gives us a very little more than three as the ratio between the 

 fin-rays and the segments. From figure 2 we obtained a little under 

 four. Both results are necessarily correct. There is considerable 

 variation in the relation between the fin-rays and the muscular seg- 

 ments and the neural arches. This is exhibited in the following table 

 of observations on a single specimen. 



The Lampreys then have advanced beyond the Myxines toward 

 the Gnathostomes as regards the structure of the median fins, in the 

 greater development and efficiency of those organs, and in the addi- 

 tion of special fin-muscles whioh seem to be wanting in the lower 

 group. And while in Myxine the original independence of the 

 median fin skeleton and the axial skeleton is shown by the e.vistence 

 of the median fin-rays before any neural arches have appeared, and 

 by their want of agreement with the musi-ular segments to which the 

 axial skeleton will conform when it does arise, in Petromyzon we 

 have the same independence more strikingly reaflirmed by the simul- 

 taneous existence of neural arches and median fin-rays, and their 

 perfect indifference to one another. 



Median Fins of Elasinobranchii.* 

 We shall here consider the skeleton of the dorsal and anal fins alone. 

 That of the caudal fin has on the ventral side undergone peculiar 

 modifications by the union of tin-rays with hnemal s[)ines. 



* The term Elasmobranchii includes the Chimaeroids with the Sharks and Rays. I 

 have had no opportunity of examining the tin skeletons of the former. But Chimaera 

 seems to me to be but a divergent form of Sharks and to have its nearest living 

 relative in Cestracion. They are quite specialized forms, Cestracion the less so. 



I will state here that the Elasmobranchs described came from Wood's Hole, Mass. 

 The names used are those given by Dr. Gill in the IT. S. Fish Coram. Rep. for 

 1871-72. 



