160 FISHES CHAP. 
the living Chondrostei and certain extinct Crossopterygii, afford 
examples of this unsymmetrical or heterocercal type of tail. A 
third type is the “homocercal.” In this type the caudal fin 
appears externally as if perfectly symmetrical, the supporting 
fin-rays radiating from the blunt extremity of the tail in such a 
way that a prolongation of the axis of the body appears to divide 
the fin into equal-sized and continuous upper and lower lobes 
(Fig. 343). Dissection, however, reveals the fact that the 
terminal portion of the vertebral column is bent upwards as in 
the heterocercal tail, and that while the dorsal lobe is almost 
vestigial, the ventral lobe is enormously developed, and its 
supporting rays so inclined backwards parallel to the axis of the 
body as to form practically the whole of the caudal fin, with the 
exception of the dorsal border, which is formed by the few 
remaining fin-rays of the dorsal lobe (Fig. 140). A homocercal 
tail, therefore, is a disguised or masked heterocereal tail. It is 
specially characteristic of Teleosts, and is closely approached in 
the Holostean genera Lepidosteus (Fig. 299) and Amia, which 
offer an interesting transition. from the heterocercal to the homo- 
cercal types; and, singularly enough, even the heterocercal tail of 
the Palaeozoic Shark Cladoselache (Fig. 249), seems as if it had 
undergone some degree of independent specialisation in the same 
direction. The homocercal tail exhibits much diversity of form in 
different Teleosts, sometimes being rounded or lancet-shaped, and 
sometimes having a deeply-forked hinder margin. One of the 
Ribbon-Fishes, Vrachypterus taenia, is singular in having the 
caudal fin on the dorsal side of the tip of the tail, and directed 
upwards like a fan. In someeTeleosts, again, there is no recog- 
nisable upward deflection of the terminal portion of the vertebral 
axis, and the caudal fin-rays séem to be derived in equal propor- 
tions from the dorsal and ventral lobes of the fin (Fig. 414). 
This apparently diphycercal tail is probably a secondary acquisi- 
tion, and may be considered due to the atrophy of the terminal 
portion of the vertebral column, and the subsequent coalescence of 
the dorsal and ventral lobes of the caudal fin round the extremity 
of a more or less abbreviated tail. It is even possible that in 
some Fishes the proper caudal fin has completely atrophied, 
and that the apparent caudal fin has really been formed by a 
similar modification affecting the hinder portions of the dorsal and 
anal fins. In the extinct Crossopterygian genera, Coelacanthus, 
