of smell, sight, taste, and hearing, which, according to the Professor, “make up the head,” originated the four cranial vertebræ which constitute the skull,—it becomes us to test the central idea, thus converted into a sort of historic myth, by the realities of actual history. What, then, let us inquire, is the real history of the cerebral development of the vertebrata, as recorded in the rocks of the earlier geologic periods? [Illustration: Fig. 7. _Osseous points of placoid cranium._[12] (Mag. twelve diameters)] Though the vertebrata existed in the ichthyic form throughout the vastly extended Silurian period, we find in that system no remains of the cranium: the Silurian fishes _seem_, as has been already said, (page 53,) to have been exclusively Placoid, and the purely cartilaginous box formed by nature for the protection of the brain in this order has in no case been preserved. Teeth, and, in at least one or two instances, the minute jaws over which they were planted have been found, but no portion of the skull. We know, however, that in the fishes of the same order which now exist, the cranium consists of one undivided piece of a cartilaginous substance, set thickly over its outer surface with minute polygonal points of bone, (fig. 7,) composed internally of star-like rays, that radiate from the centre of ossification, and that present, in consequence, seen through a microscope, the appearance of the polygonal cells of a coral of the genus Astrea. The pattern induced is that of stars set within polygons. Along the sides or top of this unbroken cranial box, that exhibits no mark of suture, we find the perforations through which the nerves of smell, sight, taste, and hearing passed from the brain outwards, and see that they have failed to originate distinct vertebral envelopes for themselves;—they all lodge in one undivided mansion-house, and have merely separate doors. We find, further, that the homotypal _ribs_ of the entire cranium consist, not of four, but simply of a single pair, attached to the occiput, and which serves both to suspend the jaws, upper and nether, in their place under the middle of the head, and to lend support to the hyoid and branchial framework; while the scapular ring we find existing, as in the higher vertebrata, not as a cerebral, but as a cervical or dorsal appendage. In the wide range of the animal kingdom there are scarce any two pieces of organization that less resemble one another in form than the vertebræ of the placoids resemble their skulls; and the difference is not merely external, but extends to even their internal construction. In both skull and vertebræ we detect an union of bone and cartilage; but the bone of each vertebra forms an internal continuous nucleus, round which the cartilage is arranged, whereas in the skulls it is the cartilage that is internal, and the bone is spread in granular points over it. If we dip the body of one