142 



Mr. K. H. Whitehouse. 



[Oct. 30, 



Lctbridm are provided with a highly specialised form, and this corresponds 

 very well with specialised features in general. 



4. Geijhyroccrcij. — Something must be said concerning the gephyrocercal 

 caudal fin, since the tendency, unfortunately, has been to depart from the 

 true definition of the term. Gephyrocercy is synonymous with secondary 

 diphycercy ; to be gephyrocercal, the tail must have lost its original caudal 

 fin, and the secondary fin have been formed by the union, round the 

 abbreviated extremity, of the rays of the dorsal and ventral median fins ; 

 moreover, it must be perfectly symmetrical externally and internally, so that 

 the dorsal and ventral rays are equal in number. 



Such a caudal fin lias been observed in two forms only among the 

 Teleostei, viz., Ficrasfer and Orthagoriscus, as Kyder has pointed out. 



Dollo* gives gephyrocercy a very wide application, attributing it to most 

 forms which have a continuous median fin, such as the eels ; but I believe 

 that all these forms can be shown to possess honiocercal caudal fins. Several 

 authors have followed Dollo's statements, and some mis-statements will 

 require correction ; for example, Mastaccmlelvs is usually considered to possess 

 a gephyrocercal caudal fin, whereas its internal structure is as typically 

 homocercal as could be desired. 



As yet, I have not been able to study the caudal fin of the Dipnoi in 

 detail, but it is highly probable that this group will be found to afford the 

 best examples of tlie gephyrocercal form. 



There are many other terms which are sometimes used in connection with 

 caudal fins, but I believe that the four terms here dealt with, viz., protocercal, 

 heterocercal, liomocercal, and gcpliyrocercal will include any type of piscine 

 caudal fin likely to be met with. 



TIT. — Morpholofiy of Paris. 



A word will ho nocossaiy on the morphology of the various parts of tlie 

 endoskeleton. Opinions are undivided on tlie nature of the urostylc ; tliis 

 structure is the result of tlit^ ftision of several vertebras which ()ccui)ied the 

 uj)turiied part of the vertebral axis. 



After a careful examination of the caudal iins of a Lugi^ number of 

 Teleosts, and also of some Elasmobranchs and Ganoids, T have come to the 

 conclusion that there is very considerable evidence that the liypurals of the 

 fin are, in general, the result of the fusion of radials or somactids with 

 hajmal arches ; apart from actual evidences, I hope to bring forward in the 

 early future strong theoretical reasons why this should be so. There is 



* "Knr l.i pliyloguiiic di's (HiiiioimteH," ' J3ulIotin do la Soci6t6 Beige do G6ologie,' etc., 

 vol. !), ]). 1)3, 1895. 



