54 DR. W. a. RIDEWOOD ON THE 



before backwards. According to this view, therefore, the fewer 

 double vertebrae in the tail in existing forms of Selachians, the 

 more specialized the fish. But Mayer, in disposing of the case 

 of Scymnus, upon which von Ihering placed so much reliance, 

 has sliowu (13. p. 265) that the idea is altogether untenable. 

 He holds that the diplospondylous condition is secondary, and is 

 due to the halving of the normal vertebrae. Hasse (7. p. 21), 

 although lending active support to von Ihering's theory, at the 

 same time regards every alternate "vertebra" of the tail as 

 intercalated, and therefore not homologous with the others. 



Embryology unfortunately throws no light upon the subject. 

 Both Gotte and von Ihering (6. p. 418, and 9. pp. 222 & 235) 

 were agreed that the ontogenetic segmentation of the caudal 

 vertebrae was precluded by the disposition of the crural and 

 intercrural plates ; and Balfour (1. p. 455) satisfied himself by 

 actual examination of embryos that the duplicity of the caudal 

 vertebrae was not due to secondary segmentation, but was 

 observable so soon as the vertebral column showed any signs of 

 differentiation into vertebrae. The figure given by Klaatsch 

 (10. p. 172, fig. 3) of the longitudinal section of the embryonic 

 vertebral column of Mustelus fully confirms the conclusions of 

 this embryologist. 



Balfour was inclined to explain the want of correspondence 

 between the metamerism of the caudal vertebrse and that of the 

 nerves and muscles by the fact that the former are differentiated 

 later than the latter. Since, however, he also showed (1. p. 453) 

 that the segmentation of the continuous cartilaginous sheath of 

 the notochord was determined by the muscle segments, and gave 

 good reasons for the fact, the lateness of the differentiation of 

 the vertebral segments cannot be taken as an explanation of 

 diplospondyly. It merely leaves open the possibility of other 

 influences coming into play and over-ruling the dominating 

 metamerism of the muscles and nerves. 



Caudal diplospondyly being so widely spread among existing 

 Sharks, and the fact that there are no traces of the actual 

 process of doubling during ontogenetic development, point to the 

 conclusion that the condition is a very ancient one. With the 

 object of ascertaining whether palaeontology could assist in the 

 solution of the problem, I availed myself of the kind assistance 

 of Mr. A. S. Woodward in examining the specimens of fossil 

 Sharks at the Natural History Museum. The results were 



