1909.] 



The Caudal Fin of Fishes. 



141 



All the dermotrichia are hypaxial, to the total exclusion of the dorsal 

 contribution to the fin. There is practically only one hypural, although this 

 is cleft distally, showing it to be composed of at least two ; the last vertebral 

 segment and the hypural become one bone, and indistinguishably fused 



Text-fig. 4. — Caudal fin of Lophius piscatorius. 

 dev., dermotrichia cut short ; 

 %•) hypural ; 

 I.V., last vertebra. 



together. The spinal cord can be traced to the end of the hypural bone. 

 A more specialised type of homocercy can scarcely be expected. 



In the three preceding types, a broad outline of the effects of specialisation 

 in homocercal tails is shown, and it will be seen that great abbreviation of 

 the axis and restriction of the caudal fin to hypaxial rays are among the chief. 

 Abbreviation is demonstrated by the excessive reduction of the urostyle and 

 the consequent concentration of the supporting elements of the rays ; but 

 also — and this is a point which does not seem to have been noticed before — 

 by the presence of the spinal cord beyond the actual termination of the 

 vertebral axis ; this last feature seems to be universal in instances of more 

 advanced homocercy. 



Practically all the gradations between the three forms of homocercy 

 described are to be found among other Teleosts. Special attention has been 

 directed to ascertain whether the various grades in specialisation corresponded 

 with the general classification which is based on specialised features in 

 general, with a view to discovering whether the structure of the caudal fin 

 could be used as a factor in classifying fishes. It has been found, however, 

 that the successive grades of specialisation in the caudal fin do not correspond 

 with the order of classification given by Boulenger in the Cambridge I^Tatural 

 History ; and hence it may be said that, in general, the caudal fin structure 

 may not be depended upon as a taxonomic feature. However, this fin need 

 not be totally ignored in this connection, and it is likely that it may be 

 useful in minor classification ; for example, in the Perciformes, the Serranidce 

 possess a comparatively lowly specialised homocercal caudal, while the 



VOL. LXXXII. — B. M 



