formed part of it,—just as the teeth of the Placoids grew out of the skin on which they rest,—the _reptile_ teeth within rested in shallow pits,—the first faint indications of true sockets. [Illustration: Fig. 14. UNDER PART OF HEAD OF OSTEOLEPIS.[16]] [Illustration: Fig. 15. HEAD OF OSTEOLEPIS, SEEN IN PROFILE.] That space included within the arch formed by the sweep of the under jaws, which we find occupied in the osseous fishes by the hyoid bones and the branchiostegous rays, was filled up externally, in the Dipterians and Cœlacanths, and in at least two genera of Cephalaspians, by dermal plates; in some genera, such as the _Diplopterus_, by three plates; in others, such as the _Holoptychius_ and _Glyptolepis_, by two; and in the _Asterolepis_, as we shall afterwards see, by but a single plate. In the _Osteolepis_ these plates were increased to five in number, by the little plates 14, 14, (fig. 14,) which, however, may have been also present in the _Diplopterus_, though my specimens fail to show them. The general arrangement was of much elegance,—an elegance, however, which, in the accompanying restorations, the dislocation of the free plates, drawn apart to indicate their detached character, somewhat tends to obscure. But the position of the eyes must have imparted to the animal a sinister reptile-like aspect. The profile, (fig. 15,) the result, not of a chance-drawn outline, arbitrarily filled up, but produced by the careful arrangement in their proper places of actually existing plates, serves to show how perfectly the dermo-skeletal parts of the creature were developed. Some of the animals with which we are best acquainted, if represented by but their cuticular skeleton, would appear simply as sets of hoofs and horns. Even the tortoise or pengolin would present about the head and limbs their gaps and missing portions; but the dermo-skeleton of the _Osteolepis_, composed of solid bone, and burnished with enamel, exhibited the outline of the fish entire, and, with the exception of the eye, the filling up of all its external parts. Presenting outside, in its original state, no fragment of skin or membrane, and with even its most flexible organs sheathed in enamelled bone, the _Osteolepis_ must have very much resembled a fish carved in ivory; and, though so effectually covered, it would have appeared, from the circumstance, that it wore almost all its bone outside, as naked as the human teeth. [Illustration: Fig. 16. CRANIAL BUCKLER OF DIPLOPTERUS.] [Illustration: Fig. 17. CRANIAL BUCKLER OF DIPLOPTERUS.]