The =Cranium= is an oblong box, with a flattened floor and a more irregular roof. Its sides are expanded in front owing to the olfactory capsules, and behind owing to the auditory capsules, while in the middle they are deeply hollowed to form the orbits. (_a_) On the dorsal surface of the cranium the following points should be noticed. First at the anterior end, the large thin-walled =nasal= or =olfactory capsules= (fig. 6, 1), each of which is drawn out into a narrow cartilaginous process. The olfactory capsules have no ventral walls, and are separated from one another by the =internasal septum=, which is drawn out into a third slender process. These three processes together constitute the =rostrum= (fig. 6, 2). Behind the olfactory capsules comes a large, nearly circular, hole, the =anterior fontanelle=, slightly behind which are the two =ophthalmic foramina=. The dorsal and ventral boundaries of the orbits are respectively formed by the prominent =supra-orbital= and =suborbital ridges=. Behind are the =auditory capsules= (fig. 6, 8), each of which is marked by a pair of prominent ridges, converging towards the middle line to a pair of apertures. These apertures communicate with two canals, the =aqueductus vestibuli=, which lead into the internal ear. The two ridges lodge respectively the =anterior and posterior vertical semicircular canals= of the ear. (_b_) The principal structures to be noted in a side view of the cranium are contained in the =orbit= or eye-cavity. Near the base of the orbit at its anterior end is seen the small =orbitonasal foramen= (fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is the large =ophthalmic foramen= (fig. 6, 5) so prominent in a dorsal view of the skull; through it the ophthalmic branches of the fifth and seventh nerves pass. Slightly further back near the ventral surface is the large =optic foramen= (fig. 6, II.) for the passage of the second nerve. Vertically above the optic foramen, near the dorsal surface, is the very small =foramen for the fourth nerve= (fig. 6, IV.). Behind and a little above the optic foramen is another small aperture, the =foramen for the third nerve=. Behind and slightly below this is the large =foramen for the sixth and main branches of the fifth and seventh nerves= (fig. 6, V.). In front of and slightly below this foramen are seen two other small apertures; the more anterior and ventral of these (fig. 6, 4) is for the passage of a vessel connecting the efferent artery of the hyoid gill with the internal carotid artery inside the skull, the more posterior and dorsal is for the =interorbital canal= (fig. 6, 3) which unites the two orbital sinuses. Above and very slightly in front of the large foramen for the sixth and main parts of the fifth and seventh nerves, are two small foramina (fig. 6, Va., and VIIa.), through which the =ophthalmic branches of the fifth and seventh nerves= enter the orbit. Behind and slightly