development of the earlier vertebrata, that made me deem them scarce less interesting than if I could. Ere, however, I attempt communicating to the reader the result of my researches, I must introduce him, in order that he may be able to set out with me to the examination of the _Asterolepis_ from the same starting-point, to the Cœlacanth family,—indisputably one of the oldest, and not the least interesting, of its order. [Illustration: Fig. 2. a. _Shagreen of the Thornback (Raja clavata.)_ b. _Shagreen of Sphagodus,—a placoid of the Upper Silurian._[9]] So far as is yet known, all the fish of the earliest fossiliferous system belonged to the placoid or “_broad plated_” order,—a great division of fishes, represented in the existing seas by the Sharks and Rays,—animals that to an internal skeleton of cartilage unite a dermal covering of points, plates, or spines of enamelled bone, and have their gills fixed. The dermal or cuticular bones of this order vary greatly in form, according to the species or family: in some cases they even vary, according to their place, on the same individual. Those button-like tubercles, for instance, with an enamelled thorn, bent like a hook, growing out of the centre of each, which run down the back and tail, and stud the pectorals of the thorn-back, (_Raja clavata_,) differ very much from the smaller thorns, with star-formed bases, which roughen the other parts of the creature’s body; and the bony points which mottle the back and sides of the sharks are, in most of the known species, considerably more elongated and prickly than the points which cover their fins, belly, and snout. The extreme forms, however, of the shagreen tubercle or plate seem to be those of the upright prickle or spine on the one hand, and of the slant-laid, rhomboidal, scale-shaped plate on the other. The minuter thorns of the ray (fig. 2, _a_) exemplify the extreme of the prickly type; the fins, abdomen, and anterior part of the head of the spotted dog-fish (_Scyllium stellare_) are covered by lozenge-shaped little plates, which glisten with enamel, and are so thickly set that they cover the entire surface of the skin, (fig. 3, _b_,)—and these seem equally illustrative of the scale-like form. They are shagreen points passing into osseous scales, without, however, becoming really such; though they approach them so nearly in the shape and disposition of their upper disks, that the true scales, also osseous, of the _Acanthodes sulcatus_, (fig. 3, _a_,) a Ganoid of the Coal Measures, can scarce be distinguished from them, even when microscopically examined. It is only when seen in section that the distinctive difference appears. The true scale of the Acanthodes, though considerably elevated in the centre, seems to have been planted on the skin; whereas the scale-like shagreen of the dog-fish is elevated over it on an osseous pedicle or footstalk (fig. 5, _a_) as a mushroom is elevated over the sward on its stem; and the base of the