VI EXTERNAL CHARACTERS 159 
Soft rays are flexible, branched distally, and generally exhibit a 
transversely-jointed structure; when present in conjunction with 
spines they invariably he 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 Fishes the tapering caudal extremity retains its 
coicidence 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” (Figs. 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. Plewra- 
canthus), but it may be doubted if any existing Fish has a 
tail which is truly and primitively diphycercal. The Dipnoi 
(Fig. 304) and the Crossopterygu, 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 Fishes 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 (Fig. 253, A). The Ostracodermi, all 
living and nearly all extinct Elasmobranchs, the Acanthodei, 
Holocephali, some extinct Dipnoi, and amongst the Teleostomi, 
