138 



Mr. R. H. Whitehouse. 



[Oct. 30, 



with them to form a single cartilage. On the ventral side the fin-rays are-^ 

 supported for nearly the whole length of the fin by cartilages directly 

 connected with the vertebral axis ; but at the anterior end of the fin there 

 are frequently a few radials quite distinct from the hieraal arches., and 

 supporting fin-rays ; immediately following, the radials are seen to have fused 

 with the arches, and the line of fusion is often retained. The caudal fins of 

 Galeus and Acanthias are good examples. Since I believe the majority of the 

 ventral fin-ray supports to be double structures, formed by the fusion of 

 radials with h;emal arches, I have called them hypurals. 



The caudal fin of Amia has been called hemi-heterocercal ; it is perhaps- 

 the most highly specialised of heteroceical tails and closely approaches the 

 homocercal of the Teleosts. For this reason the term hemi-homocercal 

 seems to me a more suitable term. But it has an essential feature of 

 heterocercy, namely, the retention of individual vertebne to the end of the^ 

 vertebral axis. 



3. Homocercy. — Before entering on this type of caudal fin it will be 

 necessary to call attention to a proposed modification of nomenclatui'e with 

 respect to three parts of the endoskeleton.' Full reasons will be given later,, 

 and I will here merely define the terms as understood in the present paper. 



Any epaxial element, which is directly connected with the vertebral axis,, 

 and which supports one or more fin-rays distally, will be called an epural or- 

 epural bone. In the second place the structures usually spoken of by 

 previous authors as epurals will here be called dorsal caudal radials ; they 

 consist of free slender bones situated epaxially, supporting fin-rays at their 

 distal ends. The third term to draw attention to is " last vertebral 

 segment," which will be used to imply the last vertebra together with tlie 

 urostyle if such is present. 



The homocercal caudal fin, which is characteristic of the Teleostei, is to be 

 considered a specialised form of the heterocercal type. The term homocercal 

 is very broad in application ; however, a caudal fin which is externally 

 symmetrical, but in wliioh tlie majority of the fin-rays are supported 

 internally l)y hypaxial elements, and in which a urostyle is present either in 

 tlie adult or at some time during larval history, may safely be referred to aa 

 a homocercal caudal fin. 



Except for two genera, Fierroifer and Orlhagoriscua, mentioned by Kyder* 

 as gophyrocercal, it is probable that there is no case among the Teleostei, 

 wli(!r(! a (laudal fin is })resent, in whicli the tail is not liomocercal. 



'I'he aim of tlic present remarks is merely to jjresent to the reader three 



* J. A. Ryder, "Evolution of tlio h'inH of I'MsIies," ' llup. U.S. Comm. of Ki.sli and 

 FiHherieH,' 1884, pub. 188(5. 



