species,—the _Glyptolepis microlepidotus_,—were all squat, robust, strongly-built fishes, of the Dirk Hatterick or Balfour-of-Burley type; and not only in the larger specimens gigantic in their proportions, but remarkable for the strength and weight of their armor, even when of but moderate stature. The specimen of _Holoptychius nobilissimus_ in the British Museum could have measured little more than three feet from snout to tail when most entire; but it must have been nearly a foot in breadth, and a bullet would have rebounded flattened from its scales. And such was that ancient Cœlacanth family, of which the oldest of our Scotch Ganoids,—the _Asterolepis_ of Stromness,—formed one of the members, and which for untold ages has had no living representative. Let us now enter on our proposed inquiry regarding the cerebral development of the earlier vertebrata, and see whether we cannot ascertain after what manner the first true brains were lodged, and what those modifications were which their protecting box, the cranium, received in the subsequent periods. Independently of its own special interest, the inquiry will be found to have a direct bearing on our general subject. CEREBRAL DEVELOPMENT OF THE EARLIER VERTEBRATA. ITS APPARENT PRINCIPLE. It is held by a class of naturalists, some of them of the highest standing, that the skulls of the vertebrata consist, like the columns to which they are attached, of vertebral joints, composed each, in the more typical forms of head, as they are in the trunk, of five parts or elements,—the centrum or body, the two spinous processes which enclose the spinal cord, and the two ribs. These cranial vertebræ, four in number, correspond, it is said, to the four senses that have their seat in the head: there is the nasal vertebra, the centrum of which is the vomer, its spinal processes the nasal and ethmoid bones, and its ribs the _upper_ jaws; there is the ocular vertebra, the centrum of which is the anterior portion of the sphenoid bone, its spinal processes the frontals, and its ribs the _under_ jaws; there is the lingual vertebra, the centrum of which is the posterior sphenoid bone, its spinal processes the parietals, and its ribs the hyoid and branchial bones,—portions of the skeleton largely developed in fishes; and, lastly, there is the auditory vertebra, the centrum of which is the base of the occipital bone, and its spinal processes the occipital crest, and which in the osseous fishes bears attached to it, as its ribs, the bones of the scapular ring. And the cerebral segments thus constructed we find represented in typical diagrams of the skull, as real vertebræ. Professor Owen, in his lately published treatise on “The Nature of Limbs,”—work charged with valuable fact, and instinct with philosophy,—figures in his draught of the archetypal skeleton of the vertebrata, the four vertebræ of the head, in a form as unequivocally such as any of the vertebræ of the neck or body.