The placoid teeth of the carboniferous rocks show the usual forms of the order. Some of them are sharp and pointed, as those of the hybodonts; others have a smooth, rounded, or plate-like form, as in the cestracionts. The latter often show a dark brilliant surface, and might be readily enough mistaken for well-worn pebbles. In the oblong rounded teeth of _psammodus_ the surface is densely covered with minute points like grains of sand, whence the name of the genus. These teeth, when sliced and viewed under the microscope by transmitted light, exhibit a complex reticulated internal structure. Agassiz' second great Order of fishes is named Ganoid, from a Greek word signifying brightness, in allusion to the brightly enamelled surface of their dermal covering. They differ from the placoids in having their outer surface cased in a strong armature of bone, which is disposed either in the form of large overlapping plates, as among the strange tortoise-like fishes of the Old Red Sandstone, or as thick scales, which are either placed at intervals, as along the back and sides of the sturgeon, or closely imbricated, as in the stony-gar (_lepidosteus_) of the American rivers. This strong, massive skeleton constitutes in many genera the sole support of the animal framework, the inner skeleton being of a gristly cartilaginous kind, like that of the skate. On this account traces of the vertebral column are by no means abundant among the older formations. But as the ganoids form a sort of intermediate link between the placoid or gristly fishes on the one hand, and the bony fishes on the other, they are found to present in their different genera examples of both these kinds of structure. Thus, the skeleton of the sturgeon consists of a firm cartilage, out of which the vertebræ are moulded, so that this fish was at one time ranked with the sharks in the cartilaginous tribe of Cuvier. The skeletons of some of the older ganoids (as _holoptychius_), on the other hand, manifest such a decidedly osseous structure, with sometimes so much of a reptilian cast, that the bones were at first referred to some huge extinct saurians. The head of the ganoid fishes is encased in a set of large massive plates of bone, and the jaws are furnished with several rows of small sharp teeth, intermingled with a less numerous but larger-sized and more formidable kind. The interior of the mouth likewise displayed in many ancient genera groups of palatal teeth, so that the dental apparatus of these animals must have been very complex and complete. The tail in all the older ganoids was heterocercal, like that of the sharks, the lobes being not unfrequently densely covered with minute overlapping scales of bone--a peculiarity which also extended to the fins. But the fins were sometimes strengthened in another way by having the foremost ray greatly thickened and enlarged, so as to form a stiff spine like the ichthyodorulites of the placoids. The whole of the external surface