J. K. Thdcher — Median and Paired Fins. 303 



In each there is a hiyer of muscle ou each side of the cai'tilaginous 

 skeleton ; this flat mass is in each divided into separate muscles by 

 septa running from between the skeletal rods straight to the integu- 

 ment, in the way exhibited in PI, LIX, fig. 66 ; in each these muscles 

 develop a tendon in their middle plane parallel to the sides of the 

 fin ; this is inserted in the fascia over the terminal cartilages where 

 the horny fibers begin. These last are the same in nature and 

 arrangement in each. 



The skeletal elements remain (see PI. LIX, iig. 67). We have a 

 short terminal piece in each, then comes a longer middle piece. There 

 is left in the median fin a proximal row of cartilages, for the most 

 part separate, which are again much shorter than the middle pieces. 

 In the ventral fin the solid basale metapterygii and a half of the girdle 

 correspond to these in every particular, except in not being of 

 separate rays. The similarity between the two fins is complete except 

 in a single particular. And that exception would be removed by a 

 process which is familiar in both, namely concrescence. Even as they 

 stand, I think that a ventral tin with one half the girdle resembles the 

 dorsal more than it does the pectoral. 



A certain amount of similarity warrants us in inferring an earlier 

 state when the similarity was greater. It has been proved that at that 

 earlier time the median fins were composed of separate rays. The 

 greater similarity, then, can only be attained by the resolution of the 

 basale metapterygii into its component parts. 



In the Elasmobranchs, as is well known, the primordial fin-skeleton 

 is supplemented in both median and paired fins by the well kno%vn 

 horny fibers. In the higher fishes these are replaced by the dermal 

 rays. The presence of horny fibers in the adipose fin of the salmon, 

 shows that the horny fibers were the earlier form. Therefore the 

 same changes have been taking place in the median and j)aired fins 

 at the same time. The same general result, i. e., of concomitant varia- 

 tion in median and paired fins is confirmed in the sub-groups of 

 Elasmobranchs, with regard to minor changes in the primordial 

 skeleton. 



The homodynamism of median and paii'ed fins comes out strikingly 

 in Centrlna Salvkmi. Here, according to descriptions, a fold of skin 

 is raised along the median line of the back, recalling the early 

 continuous fold of skin along the back in ^iniphloxus. But similarly 

 there appear two folds of skin along the sides, recalling the continuous 

 lateral folds of Amphioxus. It is at once a proof of the homodynam- 

 ism of the two, and a confirmation of the views here presented of 

 their orisjin. 



