352 KDWIN S. GOODIJiril. 



calling in the aid of ilio theory of excalation and intercalation 

 (see above, p. 344). The pectoral fin of Spinax, with its ten 

 segments, and that of Trygon, with its fifty-seven segments, 

 cannot be strictly homologous on an}^ theory', whether the 

 extra forty-seven segments have been added in the latter 

 genus, or withdrawn in the former. 



Observations on the Structure and Development of the Fins 

 OF Elasmobranchs. 



The material used was obtained chiefly from the Plymouth 

 Laboratory of the Marine Biological Association; but I also 

 have to thank Prof. Dohrn and Mr. Adam Sedgwick for the 

 generous gift of valuable embryos. 



The lateral fold theory is founded on the similarity between 

 the median and the paired fins, yet comparatively little has 

 been published on the develo})ment and structure of the 

 median fins of Pjlasmobranchs since the pioneer work of 

 Thacher (35) and Mivart (23). 



Balfour (2) studied their development, and described the 

 origin of the cartilaginous radials from a continuous pre- 

 chondral i^late. An epoch in our knowledge of the median 

 fins dates from the appearance of an important paper by 

 Mayer (22). He there describes the development of the 

 skeleton in Pristiurus, and of the radial muscles from muscle- 

 buds, which had already been noticed by Doran (10). Atten- 

 tion is drawn to the presence of abortive buds behind the 

 dorsal fins, and the collector nerves and general neive-ploxus 

 is described in many adult forms. But Mayer was unable to 

 trace accurately the relation borne by the buds to the myotomes, 

 nor did he follow out the process of concentration in detail. 



Harrison (19) has published an excellent account of the 

 development of the median fins in Salmo ; in this fish, how- 

 ever, the conditions are somewhat different, and the concen- 

 tration much less pronounced. 



Finally, Brans has lately described some stages in the 



