Before giving a general account of the adult skull it will be well to briefly describe its development. GENERAL DEVELOPMENT OF THE CRANIUM[8]. Shortly after its appearance, the central nervous system becomes surrounded by a membranous mesodermal investment which in the region of the spinal cord is called the =skeletogenous layer= or =perichordal sheath=, while in the region of the brain it is called the =membranous cranium=. Ventral to the central nervous system is the notochord, which extends far into the region of the future cranium, and like the nervous system, is enclosed by the skeletogenous layer. The primitive cartilaginous cranium is formed by histological differentiation within the substance of the membranous cranium and always consists of the following parts: (_a_) the =parachordals=. These are a pair of flat curved plates of cartilage, each of which has its inner edge grooved where it comes in contact with the notochord. The parachordals, together with the notochord, form a continuous plate, which is known as the =basilar plate=. The basilar plate is the primitive floor below the hind- and mid-brain. In front the parachordals abut upon another pair of cartilaginous bars, the trabeculae, the two pairs of structures being sometimes continuous with one another from the first; (_b_) the =trabeculae= which meet behind and embrace the front end of the notochord. Further forwards they at first diverge from one another, and then converge again, enclosing a space, the =pituitary space=. After a time they generally fuse with one another in the middle line, and, with the parachordals behind, form an almost continuous basal plate. The trabeculae generally appear before the parachordals. They form the primitive floor below the fore-brain; (_c_) the cartilaginous =capsules= of the three pairs of =sense organs=. At a very early stage of development involutions of the surface epiblast give rise to the three pairs of special sense organs--the olfactory or nasal organs in front, the optic in the middle, and the auditory behind. The olfactory and auditory organs always become enclosed in definite cartilaginous capsules, the eyes often as in the Salmon, become enclosed in cartilaginous sclerotic capsules, while sometimes, as in mammals, their protecting capsules are fibrous. Each pair of sense capsules comes into relation with part of the primitive cranium, and greatly modifies it. Thus the auditory or periotic capsules press on the parachordals till they come to be more or less imbedded in them. Perhaps owing to the pressure of the nasal capsules the trabeculae fuse in front, and then grow out into an anterior pair of processes, the =cornua trabeculae=, and a posterior pair, the =antorbital processes=, which together almost completely surround the nasal capsules. The sclerotic capsules of the eyes greatly modify the cranium, although they never become completely united with it.