204 Proceedings of the Royal Physical Society. 



body. The embryological basis upon which this view rested 

 has long ago been shown to be unreliable, the traces of such 

 a fold in ontogeny being now known to be absent in the less 

 highly specialised Elasmobranchs, though present in such 

 forms as Torfeclo. Certain morphologists attach great 

 weight to the evidence of the paired fins in Gladoselache, but 

 it is only necessary to look at the remarkably highly 

 specialised tail of this creature to see that its motor 

 arrangements were of a highly efficient kind, and to feel 

 the gravest doubts as to any interpretation of its paired fins 

 which makes them out to be of a primitive, less specialised, 

 and therefore comparatively slightly efficient character. If, 

 in addition, oue turns to the structure of the paired fins 

 in Cladodtos Neilsoni, described by Traquair (as intermediate 

 between the biserial archipterygium of Pleuracanthids and 

 the condition found in sharks), it is difficult to avoid the 

 belief that we have still much to learn as to the structure 

 of the paired fins in Cladoselache. 



On the whole it must, it seems to me, be admitted that, in 

 the present state of our knowledge, the former existence in 

 vertebrates of a lateral fiu-fold is absolutely hypothetical, 

 and we must therefore incline towards the other view, which 

 derives the limb from a forerunner whose existence is 

 rendered probable on independent morphological grounds. 



I may recapitulate here, in a few words, the view which I 

 hold as to the derivation of the paired fins: — 



1. There were present in primitive vertebrates a series of 

 paired external or dermal gills. These existed on each of 

 the visceral arches, as is rendered probable by their vestigial 

 occurrence on arches I.-VI. in existing vertebrates. The 

 same evidence which renders it probable that the series of 

 branchial clefts once was more extensive than it is in exist- 

 ing forms points to the probability of the series of external 

 gills once also having extended back beyond arch VI. 



2. These external gills, while primarily respiratory, were 

 highly muscular, and therefore potentially motor organs. 



3. Two pairs of these organs lost their respiratory and 

 developed their supporting (as in " balancer " of Urodeles — 

 the external gill of arch I.) and motor functions, and became 



