1910.] CAUDAL FIN OF THE TELEOSTOMI. 621 



Reference to fig. 1 (PI. XL VII.) will render a full descriptiou 

 unnecessary ; this form may be considered quite typical except in 

 possessing what may be regarded as an incipient urostyle at the 

 extreme end of the axis. It should be observed tha,t on the 

 dorsal side the separation of the radials from the neural arches 

 anteriorly, and their gra.dual approach to and ultimate fusion 

 with them posteriorly, suggests that when the median fins were 

 continuous there was a gradual ti-ansition, from be'fore backwards, 

 from widely sej^arated radials to fusion of radials and arches. 

 There is evidence tha.t the same thing occurred veuti-ally, but the 

 fusion of radials and arches has taken place much further forward 

 owing to the fact that the enlarged lobe required a much firmer 

 base of support; several Eiasmobranchs retain the line of fusion 

 in the adult (e. g. Galeus and Acanthias). 



HoMocERCY. — The homocercal type of caudal fin is characteristic 

 of the Teleostei, and is a specialized form of heterocei'cy. Though 

 the term is very broad in application, a tail-fin which is externally 

 symmetrical, but in which the majority of the fin-rays are 

 supported internally by h5rpaxial elements, and in which a true 

 urostyle is present either in the adult or some time during larval 

 history, may safely be i-eferred to as a homocercal caudal fin. 



Special attention has been directed towards ascertaining which 

 features may be used to determine the degree of specialization 

 exhibited by the homocercal caudal fin ; these characters may be 

 enumerated as follows : — 



I. Characters indicative of a low degree of specialization : — 



{a) Notochord exserted beyond the actual vertebral axis . 

 (6) Long urostyle, not incorporated with an adjacent hypural 



bone, 

 (c) Vestiges of fin-ray supports of the nature of opisthural 



cartilages. 

 {d) Large luimber of hypural bones associated with the last 



vertebral segment. 



Clupea may be quoted as an example of this type. 



II. Characters indicative of a high degree of sjieciaUzation : — 



(a) Greatl}'^ reduced urost3de, fused with a hypural bone. 



(b) Reduction of the number of hypurals dissociated with the 



last vertebral segment. 



Among the caudal fins examined there is considerable evidence 

 forthcoming to substantiate the view that hypurals and epnrals 

 are formed by the fusion of vertebral arches and radials, and tlie 

 following may be quoted as affording examples : — Acanthias, 

 Galeus, Heteivdontus, Acipe)i.ser, Polyodon, Synodoitis, Plotosus, 

 Ajigailla, Conger, Gadus, Gadicidas, J/olva, Jlotella, Centriscus, 

 Belone, Box, Zeus, Flearoncctes. 



Proc. ZooL. Soc— 1910, No. XLI. 41 



