the Teleostean Caudal Fin, 4.27 



from that of the Elopidae in that the last two centra have 

 aborted, and the anterior uroneural is ankylosed with the 

 actual last centrum (corresponding to the third last of the 

 Elopidae)." The same author says further: — "I have 

 already mentioned Mr. Whitehouse's paper on the caudal fin 

 of fishes, and I have shown that the element which he terms 

 ' urostyle ' in Glupea is formed by one or more displaced 

 posterior neural arches or ' uroneurals ' ; the homocercal 

 caudal fin should not then be defined by the presence of a 

 urostyle formed by the fusion of upturned vertebne, but by 

 the modification of posterior neural arches into uroneurals 

 which functionally replace and so lead to the suppression of 

 the centra of the upturned vertebrae." 



From the latter quotation the reader might be inclined to 

 suppose that the ankylosis of a " uroneural " with one of the 

 terminal vertebrae constitutes what I called a urostyle ; and 

 for this reason it will be necessary for me to emphasize that 

 I consider a urostyle to be the result of a fusion of vertebral 

 centra only, and that anything of the nature of neural arches 

 does not at all enter into the formation of a urostyle. In the 

 case of Clupea I do not think " that the last two centra have 

 aborted," but that several centra which once formed the 

 upturned portion of the vertebral column have become united 

 and have formed a single rod-like bone or urostyle. A 

 reference to the figure of Mr. E. T. Newton, F.R.S.*, shows 

 that in the young Clupea sprattus the upturned part of the 

 axis consists of distinct centra which later on lose their 

 individuality and fuse together to form a urostyle. Moreover, 

 in Clupea, the neural arches of the centra which have formed 

 the urostyle still persist in the form of a triangular bone 

 resting upon the urostyle. 



I do not consider the persistence of posterior neural arches 

 in the form of "uroneurals " as functionally replacing the 

 centra of the upturned vertebrae, but rather that they persist 

 in order to still carry out their function of protecting the 

 spinal cord, which seems always to extend to the distal end 

 of the last hypural, even when the actual vertebral column 

 terminates at the proximal end of the terminal hypural 

 bones. 



With reference to the caudal fin of Fierasfer, Mr. Regan 

 finds it impossible to regard it as gephyrocercal, apparently 

 for the reason that the rays which may be considered to 

 constitute the caudal fin are not supported by radials, as the 

 dorsal and anal fins are. The same author considers the 



* ' Journal of the Quekett Micros. Club,' 1882, ser. ii. vol. i. p. 79. 



2y* 



