542 POPULAR SCIENCE MONTHLY. 



Mollier, a supporter of the lateral-fold view, is to the effect that it does 

 not occur in such ordinary sharks as Pristiurus and Mustelus, while it 

 is to be gathered from Balfour himself that it does not occur in Scyl- 

 l iu m ( Scylliorhinus ) . 



"It appears to me that the knowledge we have now that the longi- 

 tudinal ridge is confined to the rays and absent in the less highly special- 

 ized sharks, greatly diminishes its security as a basis on which to rest 

 a theory. In the rays, in correlation with their peculiar mode of life, 

 the paired fins have undergone (in secondary development) enormous 

 extension along the sides of the body, and their continuty in the embryo 

 may well be a mere foreshadowing of this. 



' ' An apparently powerful support from the side of embryology came 

 in Dohrn and Eabl 's discoveries that in Pristiurus all the interpterygial 

 myotomes produce muscle buds. This, however, was explained away by 

 the Gegenbaur school as being merely evidence of the backward migra- 

 tion of the hind limb — successive myotomes being taken up and left 

 behind again as the limb moved further back. As either explanation 

 seems an adequate one, I do not think we can lay stress upon this body 

 of facts as supporting either one view or the other. The facts of the 

 development of the skeleton can not be said to support the fold view; 

 according to it we should expect to find a series of metameric supporting 

 rays produced which later on become fused at their bases. Instead of 

 this we find a longitudinal bar of cartilage developing quite continu- 

 ously, the rays forming as projections from its outer side. 



"The most important evidence for the fold view from the side of 

 comparative anatomy is afforded by : ( 1 ) The fact that the limb derives 

 its nerve supply from a large number of spinal nerves, and (2) the 

 extraordinary resemblance met with between the skeletal arrangements 

 of paired and unpaired fins. The believers in the branchial-arch 

 hypothesis have disposed of the first of these in the same way as they 

 did the occurrence of interpterygial myotomes, by looking on the nerves 

 received from regions of the spinal cord anterior to the attachment of 

 the limb as forming a kind of trail marking the backward migration of 

 the limb. 



"The similarity in the skeleton is indeed most striking, though its 

 weight as evidence has been recently greatly diminished by the knowl- 

 edge that the apparently metameric segmentation of the skeletal and 

 muscular tissues of the paired fins is quite secondary and does not at all 

 agree with the metamery of the trunk. What resemblance there is may 

 well be of a homoplastic character when we take into account the sim- 

 ilarity in function of the median and unpaired fins, especially in such 

 forms as Raja where the anatomical resemblances are especially striking. 

 There is a surprising dearth of paleontological evidence in favor of this 

 view. ' ' 



