l6o FISHES CHAP. 



the living Chondrostei and certain extinct Crossopterygii, afford 

 examxjles of tliis 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 heterocercal 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 

 Eibbon-Fishes, Trachypterus 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 some Teleosts, again, there is no recog- 

 nisable upward deflection of the terminal portion of the vertebral 

 axis, and the caudal fin-rays seem 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, Coelacanthv.s, 



