and the older ichthyologists; for it is a classification founded, not on the character of the internal but on that of the cuticular or dermal skeleton. And while to the geologist it possesses great and obvious advantages over every other,—for of the earlier fishes very little more than the cuticular skeleton survives,—it has this further recommendation to the naturalist, that, (in so far at least as its author has been true to his own principles,) instead of anomalously uniting the highest and lowest specimens of their class,—the fishes that most nearly approximate to the reptiles on the one hand, and the fishes that sink furthest towards the worms on the other,—it gathers into one consistent order all the individuals of the higher type, distinguished above their fellows by their development of brain, the extensive range of their instincts, and the perfection of their generative systems. Further, the history of animal existences, as recorded in the sedimentary rocks of our planet, reads a recommendation of this scheme of classification which it extends to no other. We find that in the progress of creation the fishes _began to be_ by groupes and septs, arranged according to the principle on which it erects its orders. The Placoids came first, the Ganoids succeeded them, and the Ctenoids and Cycloids brought up the rear. The march has been marshalled according to an appointed programme, the order of which it is peculiarly the merit of Agassiz to have ascertained. Now, may I request the reader to mark, in the first place that what we have specially to deal with at the present stage of the argument are the Placoid fishes of the Silurian formations, Upper and Lower. May I ask him to take note, in the second, that the long-fronted _chondropterygian_ series of Cuvier, though it includes, as has already been said, the Placoid order of Agassiz,—just as the red-blooded division of animals includes the bimana and quadrumana,—is no more to be regarded as _identical_ with the Placoids, than the red-blooded animals are to be regarded as identical with the apes or with the human family. It simply includes them in the character of _one_ of the three great divisions into which it has been separated,—the division ranged, if I may so express myself, on the extreme right of the line; its middle portion, or main body, being composed of the _Sturiones_, a family on the general level of the osseous fishes; while, ranged on the extreme left, we find the low division of the _Suctorii_, _i. e._ Cyclostomi, or Lampreys. But with the middle and lower divisions we have at present nothing to do; for of neither of them, whether _Sturiones_ or _Suctorii_, does the Silurian System exhibit a trace. Further be it remarked, that the scheme of classification which gives an abstract standing to the _Chondropterygii_, is in itself merely a certain perception of resemblance which existed in certain minds, having _cartilage_ for its general idea; just as