1906. j SELACHIAN FISHES. 729 



regarded as formed by concrescence of the proximal segments of 

 the radials articulating with the pectoral arch. 



The innervation of the paired fins in Mustelus eanis was 

 studied by Thacher, who found that the pectoral fin was inner- 

 vated by branches of the first fifteen myelonal nerves. The 

 posterior four (12-15) ran direct to the metapterygium and 

 then divided into dorsal and ventral branches ; the next four 

 (8-11) formed a plexus, from which they emerged and then 

 behaA^'ed in a similar manner to the ones behind them ; the first 

 seven united with each other and with a minute branch of the 

 vagus to form a cord which passed through the foramen of 

 the pectoral arch and then divided to form a dorsal and a ventral 

 branch. In the pelvic fin a considerable number of nerves 

 anterior to the fin took part in forming a cord which passed 

 through the foramen of the pelvis, and posterior nerves I'an to 

 the basipterygium and bif ui'cated in the same way as the posterior 

 nerves of the pectoral fin *. 



In the Chondrostei the dermal fin-rays are more or less ossified 

 and segmented ; except for this, the median fins were found to be 

 exactly similar to those of generalised Selachians, and the pelvic 

 fins to be of a more primitive type than the Selachian pelvics, 

 and to bear even a closer resemblance to the dorsal and anal fins, 

 inasmiich as the jDOsterior basals remained separate, and the pelvis, 

 formed by the union of the anterior ones, did not meet its 

 fellow of the other side. 



Thacher's main results may be stated as follows : — In Selachii 

 and Chondrostei both median and paired fins consist of dermal 

 rays, muscles, and endoskeletal supports which are similar in 

 structure and in relative position. The pectoral and pelvic 

 girdles must have been formed by fusion and subsequent out- 

 growth of the anterior basalia. Both median a-nd paii-ed fins 

 are to be regarded as derived from originally continuous fins, 

 perhaps homologous with the median fin-folds and metaplem-al 

 ridges of Aniphioxus. The type of fin termed '■ archipterygium " 

 by Gegenbaur must be secondary, and the suggested homology of 

 limb-girdles with gill-arches cannot be seriously entertained. 



Thacher must be held to have proved his case from the facts 

 of comparative anatomy alone ; but, even if it be granted that 

 the extraordinary resemblances between median and paired fins 

 might possibly have arisen in organs of dissimilar origin, the 

 proofs of so improbable a hypothesis must be substantial. If they 

 wish to be taken seriously, supportei-s of the theory of the 

 dissimilar origin of median and paired fins must bring forward 

 evidence to show that this similar structure is secondary ; and if 

 that be the case we should expect to find signs of it in their 



* Whilst the Gegenbaurian school have explained the innervation of the pelvic 

 fins as due to their migi-ation, the3' have all ignored the similar innervation of the 

 pectoral fin, which is absolutely inexplicable in terms of their hypothesis. Moreover, 

 since forward migration of the pelvic fins in Teleostei has not led to their innervation 

 by a number of spinal nerves belonging to the myotomes through which the}'' have 

 passed, why should such an effect have been produced by their backward migration ? 



