first described in the “Edinburgh Review” for 1845 by a vigorous writer and masterly geologist, (generally understood to be Professor Sedgwick of Cambridge,) as “a characteristic portion of a fish undoubtedly belonging to the Cestraciont family of the Placoid order.” In the “American Journal of Science” for 1846, Professor Silliman figured, from a work of the States’ Surveyors, the defensive spine of a Placoid found in the Onondago Limestone of New York,—a rock which occurs near the base of the Upper Silurian System, as developed in the western world;[23] and in the same passage he made reference to a mutilated spine detected in a still lower American deposit,—the Oriskany Sandstone. In the Geological Journal for 1847, it was announced by Professor Sedgwick, that he had found “_defences of fishes_” in the Upper Llandeilo Flags, and by Sir Roderick Murchison, that the “defence of an _Onchus_” had been detected by the geologists of the Government survey, in the Limestone near Bala. Sir Roderick referred in the same number to the remains of a fish found by Professor Phillips in the Wenlock Shale. And such, up to the present time, is the actual _amount_ of the evidence with which we have to deal, and the dates of its piecemeal production. Let us next consider the _order of its occurrence in the geologic scale_. { +-----+ { Upper | | Fish, 1838, { Ludlow. | 1 | (Murchison.) { | | Fish, 1842, { +-----+ (Phillips.) { Aymestry | | { Limestone. | 2 | { +-----+ UPPER SILURIAN ROCKS. { Lower | | { Ludlow. | 3 | { +-----+ { Wenlock | | Fish, 1845, { Limestone. | 4 | (Sedgwick.) { | | Fish, 1846, { +-----+ (Silliman.) { Wenlock | 5 | Fish, 1847. { Shale. | | (Phillips.) +-----+ ----- { +-----+ { Caradoc | | { Sandstone, | 6 | { &c. | | LOWER SILURIAN ROCKS. { +-----+ { Llandeilo | | Fish, 1847, { Flags, &c. | 7 | (Sedgwick.) { +-----+ ----- { +-----+ { Plynlimmon | | { Group. | _a_ |