2. those developed in connection with the special sense organs; 3. those forming the upper jaw and suspensorial apparatus. Both cartilage and membrane bones take part in the formation of the skull, and a considerable amount of cartilage remains unossified, especially in the ethmoidal and sphenoidal regions. 1. THE CRANIUM PROPER OR BRAIN-CASE. The cartilage and membrane bones of the brain-case when taken together can be seen to be more or less arranged in three rings or segments, called respectively the occipital, parietal, and frontal segments. The =occipital segment= is the most posterior of these, and consists of four cartilage bones, the =basi-occipital=, the two =exoccipitals= and the =supra-occipital=; these bound the =foramen magnum=. The =basi-occipital= (figs. 38 and 39, 5) lies ventral to the foramen magnum and only bounds a very small part of it; it forms one-third of the =occipital condyle= by which the skull articulates with the atlas vertebra. It unites dorsally with the exoccipitals and anteriorly with the basisphenoid. The =exoccipitals= are rather small bones, which form the sides and the greater part of the floor of the foramen magnum, and two-thirds of the occipital condyle. Laterally each is united with the pterygoid and opisthotic of its side. At the sides of the occipital condyle each exoccipital is pierced by a pair of foramina, the more dorsal and posterior of which transmits the hypoglossal nerve. The =supra-occipital= (fig. 39, 14) is a larger bone than the others of the occipital segment. It forms the upper border of the foramen magnum and is drawn out dorsally into a large crest which extends back far beyond the occipital condyle. In the adult the supra-occipital is completely ankylosed with the epi-otics. The =Parietal segment=. The ventral portion of the parietal segment is formed by the =basisphenoid= (figs. 38 and 39, 4) which lies immediately in front of the basi-occipital. A triangular portion of it is seen in a ventral view of the skull, but it is quickly overlapped by the pterygoids. It gives off dorsally a pair of short processes which meet the pro-otics. The alisphenoidal region is unossified and the only other constituents of the parietal segment are the _parietals_ (fig. 39, 1). These are large bones which, after roofing over the cranial cavity, extend upwards and become expanded into a pair of broad plates which unite with the squamosal and bones of the frontal segment to form a wide, solid, false roof to the skull. Each also sends ventralwards a plate which meets an upgrowth from the pterygoid and acts as an alisphenoid. The =Frontal segment=. Of the frontal segment the basal or presphenoidal and lateral or orbitosphenoidal portions do not become ossified, the dorsal portion however includes three pairs of membrane bones, the _frontals_, _prefrontals_ and _postfrontals_.