The occipital segment is formed of four cartilage bones, the =basi-occipital= below, two =exoccipitals= at the sides, and the =supra-occipital= above. The parietal segment is formed of the =basisphenoid= below, two =alisphenoids= at the sides and two membrane bones, the _parietals_ above, and the frontal segment in like manner consists of the =presphenoid= below, the two =orbitosphenoids= at the sides, and two membrane bones, the _frontals_, above. The parietals and frontals, being membrane bones, are not comparable to the supra-occipital, in the way that the presphenoid and basisphenoid are to the basi-occipital. The cartilage bones of the occipital segments are derived from the parachordals of the embryonic skull, those of the parietal and frontal segments from the trabeculae. In front of the presphenoid the basicranial axis is continued by the =mesethmoid=. (2) THE SENSE CAPSULES. These enclose and protect the special sense organs. (_a_) =Auditory capsule.= The basisphenoid is always continuous with the basi-occipital, but the alisphenoid is not continuous with the exoccipital as the =periotic= or =auditory capsule= is interposed between them. Each periotic capsule has three principal ossifications; an anterior bone, the =pro-otic=, a posterior bone, the =opisthotic=, and a superior bone, the =epi-otic=. These bones may severally unite, or instead of uniting with one another they may unite with the neighbouring bones. Thus the epi-otic often unites with the supra-occipital, and the opisthotic with the exoccipital. Two other bones developed in the walls of the auditory capsule are sometimes added, as in Teleosteans; these are the =pterotic= and =sphenotic=. (_b_) =Optic capsule.= The eye is frequently enclosed in a cartilaginous sclerotic capsule, and in this a number of scale-like bones are often developed. Several membrane bones are commonly formed around the orbit or cavity for the eye. The most constant of these is the _lachrymal_ which lies in the anterior corner; frequently too, as in Teleosteans, there is a _supra-orbital_ lying in the upper part of the orbit, or as in many Reptiles, a _postorbital_ lying in the posterior part of the orbit. (_c_) =Nasal capsule.= In relation to the nasal capsules various bones occur. The basicranial axis in front of the presphenoid is ossified, as the =mesethmoid=, dorsal to which there sometimes, as in Teleosteans, occur a _median ethmoid_ and a pair of =lateral ethmoids=[9]. Two pairs of membrane bones very commonly occur in this region, viz. the _nasals_ which lie dorsal to the mesethmoid, and the _vomers_ (sometimes there is only one) which lie ventral to it. The part of the skull lying immediately in front of the cranial cavity and in relation to the nasal capsules constitutes the =ethmoidal region=.