336 EDWIN S. GOODRICH. 



on all tlie trunk myotomes situated between the pectoral 

 and the pelvic fins in such sharks as Pristiurus and Scyllium, 

 in which these fins are widely separated in the adult (Dohrn 

 10, Rabl 31, Braus 4, and p. 343 below). IMnny of the inter- 

 mediate buds seem to disappear entirely during development. 

 In those segments which are near the fins the buds become 

 better developed and more persistent, and a large number 

 pass into the fin-fold. Muscle buds are also found in front of 

 the pectoral fin and behind the pelvic fin, dwindling in size as 

 they are farther removed from the fin-base. Thus, in these 

 sharks, the muscles of the paired fins are formed by the great 

 development in two regions of a continuous series of muscle 

 buds, vanishing posteriorly behind the cloaca. The manner 

 in which these vestigial buds disappear by redttction at 

 either end of the fin rudiment, and in which the persistent 

 buds become concentrated at the relatively narrowing 

 base of the fin, has been admirably described and figured by 

 Mollier (24) and Braus. 



The fin-base of the adult occupies much less space relatively 

 than the fin -fold of the embryo. 



Now, the radial fin-muscles being developed from buds of 

 the myotomes, naturally receive their motor nerve-supply 

 from the venti-al roots of the spinal nerves, and these corre- 

 spond in number to the myotomes which share in the forma- 

 tion of the musculature. Owing to concentration, the nerves 

 are found to converge toward the base of the fin. In front 

 and behind the nerves may be drawn together so as to form 

 a "collector^' nerve or compound stem. 



It is part of the lateral fold theory to suppose that the 

 endoskeleton of the paired fins has been derived from a series 

 of cartilaginous rods, radials, or somactidia, similar to those 

 of the median fins (Thacher 35, Mivart 23). The various 

 types of fin-skeleton, with their cartilage rays and basal pieces, 

 would have been developed from such originally segmental 

 radials by concentration and fusion. To this contention it is 

 objected that in development the radials of Elasmobranchs 

 arise in a continuous rudiment — a plate of procartilaginous 



