represented with excellent effect in another, as not merely a connecting link in the upward march of progression between the tail of the true fish and that of the true reptile, but as actually containing in itself—as the caterpillar contains the future pupa and butterfly—the elements of the reptilian and mammiferous tail. If there be any virtue in analogy, the heterocercal tail is, I repeat, of a decidedly higher type than the homocercal one. It furnishes the first example in the vertebrata of the coccygeal vertebræ diminishing to a point, which characterizes not only all the higher reptiles, but also all the higher mammals, and which we find represented by the _Os coccygis_ in man himself. But to this special point I shall again refer. With regard to that rudimentary state of the _occipital_ framework of the Placoids to which the author of the “Vestiges” refers, it may be but necessary to say that, notwithstanding the simplicity of their box-like skulls, they bear in their character, as cases for the protection of the brain, at least as close an analogy to the skulls of the higher animals, as those of the osseous fishes, which consist usually of the extraordinary number of from sixty to eighty bones,—a mark—the author of the “Vestiges” himself being judge in the case—rather of inferiority than the reverse. “Elevation is marked in the scale,” we find him saying, “by an animal exchanging a multiplicity of parts serving one end, for a smaller number.” The skull of a cod consists of about thrice as many separate bones as that of a man. But I do not well see that in this case the fact either of _simplicity_ in excess or of _multiplicity_ in excess can be insisted upon in either direction, as a proper basis for argument. Nearly the same remark applies to the maxillaries as to the skull. The under jaw in man consists of a single bone; that of the thornback—if we do not include the two suspending _ribs_, which belong equally to the upper jaw—of two bones, (the number in all the mammiferous quadrupeds:) that of the cod of four bones, and, if we include the suspending _ribs_, of twelve. On what principle are we to hold, with _one_ as the representative number of the highest type of jaw, that _two_ indicates a lower standing than _four_, or _four_ than _twelve_? In reference to the further statement, that in many of the ancient fishes “traces can be observed of the muscles having been attached to the external plates, strikingly indicating their low grade as vertebrate animals,” it may be answer enough to state, that the peculiarity in question was not a characteristic of the _most_ ancient fishes,—the Placoids of the Silurian system,—but of some Ganoids of the succeeding systems. The reader may remember, as a case in point, the example furnished by the nail-like bone of _Asterolepis_, figured in page 111, in which there exists depressions resembling that of the round ligament in the head of the quadrupedal thigh-bone.