1909.] 



TJie Caudal Fin of Fishe,^. 



137 



.are much rarer, but Amia may be considered to have an externally 

 symmetrical tail-fin ; the external form of Polyodon, too, is a very close 

 approximation to symmetry ; however, internal asymmetry seems to be 

 universal in these cases. The comlition concerning the presence of distinct 

 centra to the end of the axis seems to have several exceptions, e.g., Hctero- 

 dontds ; a close examination of the extremity of the axis will show a 

 comparatively elongated cone equal in lengtli to several of the preceding 

 vertebnt ; this shows no sign of having been separate vertebrae previously, 



na. 



v.cr. 



Text-fig. 1. — Caudal fin of Heterodcmtus philUpi. 

 d.c.r., dorsal caudal radial ; 

 h.a., haemal arch ; 

 hy., liypural ; 

 n.a., neural arch. 

 v.c.i:, ventral caudal radial. 



and thus it is not safe to conclude that it is a urostyle ; however, some may 

 prefer to regard it as an incipient urostyle, but it never occupies anything 

 but a very insignificant part of the upturned axis. 



Concerning the endoskeletal supports of the heterocercal caudal fin, on the 

 dorsal side, immediately above a series of neural arches, is a series of radials 

 (text-fig. 1, d.c.r.) supporting the fin-rays ; for the greater part of the caudal 

 fin, these radials remain distinct from the arches, as do those of the dorsal 

 fin in front, though to a greater extent, but towards the extremity they fuse 



