That space comprised within the arch of the lower jaws, in which the hyoid bone and branchiostegous rays of the osseous fishes occur, was filled by a single plate of great size and strength, and of singular form, (fig. 36;) and to this plate, existing as a steep ridge running along the centre of the interior surface, and thickening into a massy knob at the anterior termination, that nail-shaped organism, which I have described as one of the most characteristic bones of the _Asterolepis_, belonged. In the _Osteolepis_, the space corresponding to that occupied by this hyoid plate was filled, as shown in fig. 14, by five plates of not inelegant form; and the divisions of the arch resembled those of a small Gothic window, in which the single central mullion parts into two branches atop. In the _Holoptychius_ and _Glyptolepis_ there were but two plates; for the central mullion, _i. e._ line of division, did not branch atop; and in the _Asterolepis_, where there was no line of division, the strong nail-like bone occupied the place of the central mullion. The hyoidal armature of the latter fish was strongest in the line in which the others were weakest. Each of the five hyoid plates of the _Osteolepis_, or of the two plates of the _Glyptolepis_ or _Holoptychius_, had its own centre of ossification; and in the single plate of _Asterolepis_, the centre of ossification, as shown by the radiations of the fibre, was the nail-head. This head, placed in immediate contact with the strong boxes of bone which composed the under jaw, just where their central joining occurred, seems to have lent them a considerable degree of support, which at such a juncture may have been not unnecessary. In some of the nail-heads, belonging, it is probable, to a different species of _Asterolepis_ from that in which the nail figured in page 7, and the plate in the opposite page, occurred,—for its general form is different, (fig. 37,)—there appear well-marked ligamentary impressions closely resembling that little spongy pit in the head of the human thigh-bone to which what is termed the round ligament is attached. The entire hyoid-plate, viewed on its outer side, resembles in form the hyoid-bone,—or cartilage rather,—of the spotted dog-fish, (_Scyllium stellare_;) but its area was at least a hundred times more extensive than in the largest _Scyllium_, and, like all the dermal plates of the _Asterolepis_, it was thickly fretted by the characteristic tubercles. In the Ray, as in the Sharks, the piece of thin cartilage of which this plate seems the homologue, is a flat, semi-transparent disk; and there is no part of the animal in which the progress of those bony molecules which encrust the internal framework may be more distinctly traced, as if in the act of creeping over what they cover, in slim threads or shooting points,—and much resembling new ice creeping in a frosty evening over the surface of a pool. [Illustration: Fig. 37. NAIL-LIKE BONE OF HYOID PLATE.