136 



Mr. R. H. Whitehouse. 



[Oct. 30, 



with no qualification regarding its primitive or secondary symmetry ; thus it 

 is a little vague. Primitive diphycercy is synonymous with protocercy, and 

 secondary diphycercy with gephyrocercy, which will be dealt with later. If 

 tliis distinction were always kept in mind when speaking of caudal fins, much 

 inconvenience would be avoided. For all practical purposes, the term 

 diphycercy may be dismissed ; it may, however, be retained as applying 

 to a caudal tin which it may not be wise or possible to refer to as protocercal 

 or gephyrocercal, owing to lack of definite knowledge regarding its primitive 

 or secondary nature. 



Since all fishes first possess a protocercal tail in the embryonic condition,, 

 it seems quite safe to infer that it is the true primitive form. As all the 

 complications of the specialised caudal fin of Teleosts have to be evolved from 

 this primitive type, one of the first questions which arises is, — How were the- 

 fin-rays of the protocercal tail supported ? It is probably impossible to say ;. 

 but since the caudal fin formed only a part of an undifferentiated median firb 

 system, it is not unreasonable to conclude that they were supported in a 

 manner exactly similar to that by which the fin-rays of the rest of th& 

 median fiin were supported ; now all the rays of the median fins, with the 

 exception of the caudal, in modern fishes, are invariably supported by radials 

 or soinactids, hence it may be that radials were once the supporting elements 

 in the caudal. Embryology lends little aid in substantiating this, because' 

 before development has proceeded far, the shape of the caudal becomes- 

 modified into the heterocercal form. 



As a final word regarding protocercy, it must not be supposed that the 

 term is always associated with a continuous median fin ; during tlevelop- 

 ment, in Teleosts which have differentiated median fins, the elements of the 

 permanent differentiated fins are laid down before heterocercy is established, 

 so that thei-e is evidence for supposing that differentiation occurred before 

 heterocercy was adopted. 



2. Heterocercy. — A heterocercal caudal fin may be defined as one in whicl* 

 the extremity of the chorda is directed upwards, and as a consequence is. 

 unsymmetrical externally and internally, the ventral lobe being greater than 

 the dorsal ; and, moreover — and this is a most important feature, to whicli 

 I have not seen attention drawn — when centra are formed, these elements 

 remain distinc-t to the end of tlie axis. 



To each of these three features exceptions will be found ; for exani])le, 

 many heterocercal tails are of slender build and somewhat lash-like, and 

 oonseciuently there is frequently no appreciable upturning of the axis : 

 Chlamydose/nchus is a case in ])()int, and often the common dog-fish illustrates 

 the same thing, llegarding the externiilly unsymmetrical shape, exceptions 



