In the second great period of ichthyic existence,—that of the Old Red Sandstone,—we find the first example, in the class of fishes, of “monstrosity through _displacement_ of parts,” and apparently also—in at least two genera, though the evidence on this head be not yet quite complete—of “monstrosity through _defect_ of parts.” In all the Ganoids of the period, with (so far as we can determine the point) only two exceptions, the scapular bases of the fore limbs are brought forward from their typical place opposite the base of the cervical vertebræ, and stuck on to the occipital plate. There occurs, in consequence, in one great order of the ichthyic class, such a departure from the symmetrical type as would take place in a monster example of the human family in whom the neck had been annihilated, and the arms stuck on to the back of the head. And in the genera _Coccosteus_ and _Pterichthys_ we find the first example of degradation through _defect_. In the _Pterichthys_ the _hinder_ limbs seem wanting, and in the _Coccosteus_ we find no trace of the _fore_ limbs. The one resembles a monster of the human family born without hands, and the other a monster born without feet. Ages and centuries pass, and long unreckoned periods come to a close; and then, after the termination of the Palæozoic period, we see that change taking place in the form of the ichthyic tail, to which I have already referred, (and to which I must refer at least once more,) as singularly illustrative of the progress of degradation. Yet other ages and centuries pass away, during which the reptile class attains to its fullest development, in point of size, organization, and number; and then, after the times of the Cretaceous deposits have begun, we find yet another remarkable monstrosity of displacement introduced among all the fishes of one very numerous order, and among no inconsiderable proportion of the fishes of another. In the newly-introduced Ctenoids, (_Acanthopterygii_,) and in those families of the Cycloids which Cuvier erected into the order _Malacopterygii sub-brachiati_, the hinder limbs are brought forward, and stuck on to the base of the previously misplaced fore limbs. All the four limbs, by a strange monstrosity of displacement, are crowded into the place of the extinguished neck. And such, at the present day, is the prevalent type among fishes. Monstrosity through _defect_ is also found to increase; so that the snake-like _apoda_, or feet-wanting fishes, form a numerous order, some of whose genera are devoid, as in the common eels and the congers, of only the hinder limbs, while in others, as in the genera Muræna and Synbranchus, both hinder and fore limbs are wanting. In the class of fishes, as fishes now exist, we find many more evidences of the monstrosity which results from both the misplacement and defect of parts, than in the other three classes of the vertebrata united, and knowing their geological history better than that of any of the others,