Now, in tracing the geologic history of the ichthyic tail, we find these several steps or gradations from the heterocercal to the homocercal, represented by periods and formations. The Siluran periods may be regarded as representative of that true heterocercal tail of the Placoids, exemplified in _Spinax_, (page 172, fig. 48,) and _Cestracion_, (page 177, fig. 49.) The whole caudal portion of this latter animal, commencing immediately behind the ventrals, is, as becomes a true tail, slim, when compared with its trunk; the vertebræ are of very considerable solidity; the rays mucoidal; and where the spinal column runs into the terminal fin, it takes such an upward turn as that which the horse-jockey imparts, by the process of _nicking_, to the tails of the hunter and the racehorse. And with the heterocercal tail, so true in its homologies to the tails of the higher vertebrata, we find associated, as has been shown, the true homological position of the fore limbs. With the commencement of the Old Red Sandstone the ganoidal tail first presents itself; and we become sensible of a change in the structure of the attached fin, similar to that exemplified in the caudal rays of the sturgeon. As shown by the irregularly-angular patch of scales which in all the true Cœlacanths, and almost all the Dipterians,[29] runs through the _upper_ lobe of the fin, and terminates in a point, (see fig. 50,) it must have possessed the gradually diminishing vertebræ, or a diminishing spinal cord, their analogue; but the rays, fairly _set_, as their state of keeping in the rocks certify, exist as narrow oblong plates of solid bone; and their anterior edges are strengthened by a line of osseous defences, that pass from scales into rays. And in harmonious accompaniment with this fairly _stereotyped_ edition of the ichthyic tail, we find, in the fishes in which it appears, the first instance of displacement of _limb_,—the bases of the pectorals being removed from their original position, and stuck on to the nape of the neck. It may be remarked, in passing, that in the tails of two ganoidal genera of this period,—the _Coccosteus_ and _Pterichthys_,—the analogies traceable lie rather in the direction of the tails of the Rays than in those of the Sharks; and that one of these, the _Coccosteus_, seems, as has been already intimated, to have had no pectorals, while it is doubtful whether in the _Pterichthys_ the pectorals were not attached to the shoulder, instead on the head. In the Carboniferous and Permian systems there occur, especially among the numerous species of the genus _Palæoniscus_, tails of the type exemplified by the internal angle of the tail of the sturgeon: the lozenge-shaped scales run in acutely angular patches through their upper lobes; but such is their extreme flatness, as shown by the disposition of the enamelled covering, that it appears exceedingly doubtful whether any vertebral column ran beneath;—they seem but to have