VI EXTERNAL CHARACTERS I 59 



Soft rays are flexible, branched distally, and generally exhibit a 

 transversely-jointed structure ; when present in conjunction with 

 spines they invariably lie behind the latter. The presence of 

 both kinds of fin-rays, or of soft rays only, is one of the more 

 obvious distinctions between the Teleostean groups of the Acan- 

 thopterygii and the Malacopterygii, of which the Perch and the 

 Salmon respectively are well-known examples. Powerful spines 

 are frequently developed in front of the dorsal fin in many living 

 and extinct Elasmobranchs, and, under the general term of 

 " ichthyodorulites," constitute the sole fossil remains of many 

 extinct Devonian and Carboniferous genera. 



The caudal fin and the terminal portion of the tail exhibit 

 interesting modifications which are highly characteristic of par- 

 ticular groups of Fishes. In the embryonic and early larval 

 stages of most Pishes the tapering caudal extremity retains its 

 coincidence with the axis of the body, and divides the caudal 

 fin into two equal portions, a dorsal and a ventral lobe, the two 

 being continuous round the tip of the tail ; and this condition, 

 which is certainly the most primitive, is termed " protocercal " 

 or " diphycercal " (Pigs. 238 and 309). Such a symmetrical 

 tail, as we have seen, is retained in the Cyclostomata, and was 

 also present in certain extinct palaeozoic Sharks (e.g. Pleura- 

 canthus), but it may be doubted if any existing Pish has a 

 tail which is truly and primitively diphycercal. The Dipnoi 

 (Pig. 304) and the Crossopterygii, including fossil representa- 

 tives of both groups, and perhaps a few Teleosts, seem to 

 approach this condition ; but it is by no means certain that the 

 apparent symmetry is primitive, and has not been secondarily 

 acquired. In other Pishes the terminal part of the tail, 

 including also its section of the vertebral column, is bent 

 upwards, and is fringed along its upper border by the reduced 

 dorsal lobe of the caudal fin, which, nevertheless, retains its con- 

 tinuity with the ventral lobe round the tip of the tail. The 

 latter, or rather its hinder portion, is strongly developed, but, 

 owing to the prolongation of the up-tilted caudal axis beyond it, 

 the dorsal lobe appears longer than the ventral, and hence there 

 is a marked want of symmetry between the upper and lower 

 division of the caudal fin (Pig. 253, A). The Ostracodermi, all 

 living and nearly aU extinct Elasmobranchs, the Acanthodei, 

 Holocephali, some extinct Dipnoi, and amongst the Teleostomi, 



