have essentially differed in their substance originally. [Illustration: Fig. 9. a. _Portion of caudal fin of Cheiracanthus._[13] b. _Portion of caudal fin of Cheirolepis Cummingiæ._ (Mag. three diameters.)] Now, in the _Cheiracanthus_ we detect the first faint indications of a peculiar arrangement of the dermal skeleton, in relation to certain parts of the skeleton within, which—greatly more developed in some of its contemporaries—led to important results in the general structure of these Ganoids, and furnishes the true key to the character of the early ganoid head. In such of the existing Placoids as I have had an opportunity of examining, the only portions of the dermal skeleton of bone which conform in their arrangement to portions of the interior skeleton of cartilage, are the teeth, which are always laid on a base of skin right over the jaws: there is also an approximation to arrangement of a corresponding kind, though a distant one, in those hook-armed tubercles of certain species of rays which run along the vertebral column; but in the shagreen by which the creatures are covered I have been able to detect no such arrangement. Whether it occurs on the fins, the body, or the head, or in the scale form, or in that of the prickle, it manifests the same careless irregularity. And on the head and body of the _Cheiracanthus_, and on all its fins save one, the shagreen-like scales, though laid down more symmetrically in lines than true shagreen, manifested an equal absence of arrangement in relation to the framework within. On that one fin, however;—the caudal,—the scales, passing from their ordinary rhomboidal to a more rectangular form, ranged themselves in right lines over the internal rays, (fig. 9, _a_,) and imparted to these such strength as a splint of wood or whalebone fastened over a fractured toe or finger imparts to the injured digit,—a provision which was probably rendered necessary in the case of this important organ of motion, from the circumstance that it was the only fin which the creature possessed that was not strengthened and protected anteriorly by a strong spine. In the _Cheirolepis_,—a contemporary fish, characterized, like its cogeners the _Cheiracanthus_ and _Diplacanthus_, by shagreen-like scales, but in which the spines were wanting,—we find a farther development of the provision. In all the fins the richly-enamelled dermal-covering was arranged in lines over the rays, (fig. 9, _b_;) and the scale, which assumes in the fins, like the scales on the tail of the _Cheiracanthus_, though somewhat more irregularly, a rectangular shape, is so considerably elongated, that it assumes for its normal character as a scale, that of the joint of an external ray. A similar arrangement of external protection takes place in this genus over the bones of the head; the cartilaginous jaws receive their osseous dermal covering, and, with these, the hyoid bones, the opercules, and the cranium. And it is in these dermal plates,