1910.] CAUDAL FIX or THE TKLKOSTOMI. 623 



caudal fin may be considered to be a posterior anal— or, in other 

 words, a more anterior part of the ventral median fin — broufjht 

 into a terminal position by the great abbreviation and upturning 

 of the chorda! axis. 



Gephyroc'ERCY. — A gephyi'ocercal caiidnl fin may be defineil as 

 one which is quite symmetrical, externally and internally, lint 

 with symmetry secondarily acquired. Fierasfer dentatus may be 

 quoted as a typical example. Many foi-ms which are generally 

 considered gephyrocercal (e. g. J/astacembelas and Aiigmlla) are 

 not so, but cleai-ly homocercal. It is worth noticing that tliere is 

 no embryological evidence that heterocercy preceded the gepliyro- 

 cercal condition. 



Maiiy other terms are used in works dealing with the caudal 

 fin, but the four terms here used — protocercal, heterocercal, homo- 

 cercal, and gephyrocercal, understood as defined here — will probably 

 include any type of piscine caudal fin. A common term not 

 insisted on in the present paper is diphycercy, which implies 

 perfect symmetry, externally and internally ; it tends to be 

 ambiguous, for symmetry may be either primitive or secondary, 

 and primitive diphycercy corresponds to protocercy, while 

 secondary diphycercy corresponds to gephyrocercy. However, 

 diphycercy may sometimes prove useful to apply to a truly 

 symmetrical fin about which any doubt may exist as to its 

 primitive or secondary nature. 



Taxoxomic Importance. — The structure of the caudal fin 

 cannot be considered a safe criterion for taxonomic purposes, 

 though it may be useful in this connection among the smaller 

 divisions, as is well illustrated by the Acanthopterygii. 



5. Additioxal Note. 



Since the foregoing paper was written. Professor Dollo has 

 drawn my attention to his memoir on the fisiies of the ' Belgica ' 

 Expedition of 1897-99 (see Literature, ref. 11), where he advocates 

 a double nomenclature for the caudal fin. Briefly, this author 

 considers it necessary to adopt two terms — (1) physiological, and 

 (2) morphological. This may be conveniently summarized as 

 follows : — 



I. Physiological (or Ethological), which refers to the 



external form of the fin in its relation to the mode 

 of life. 



(1) Rhipidocercal., implying an expanded or fan-shaped 



form. 



(2) Gephyrocercal, implying a tapering form. 



II. Morphological, which refers to the internal structure 



of the fin. 

 (1) Diphycercfd (or Protncercal). 



(2) Heterocercal. 

 (8) Homocercal. 



41' 



