The caudal fin and the terminal portion of the tail exhibit interesting modifications which are highly characteristic of particular groups of Fishes. In the embryonic and early larval stages of most Fishes 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" (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. _Pleuracanthus_), but it may be doubted if any existing Fish has a tail which is truly and primitively diphycercal. The Dipnoi (Fig. 304) and the Crossopterygii, including fossil representatives 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 continuity 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, {160}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 heterocercal tail. It is specially characteristic of Teleosts, and is closely approached in the Holostean genera _Lepidosteus_ (Fig. 299) and _Amia_, which offer an