SKULL. 135 



Esox), and the prootic (PrO), which surrounds the anterior semi-- 

 circular canal and is pierced for the exit of the trigeminal nerve. 

 There is also an external bone, the squamosal (pterotic) (Sq), to 

 which the hyomandibular is articulated. The lower surface of the 

 cranial capsule is covered by the long parasphenoid (Ps). The lateral 

 walls of the skull are formed by two pairs of wing-like bones 

 the orbitosphenoids and the alisphenoids (fig. 586). Of these the 

 alisphenoids are applied to the sides of the parasphenoid, and are 

 almost always discernible with their openings for the exit of the 

 optic nerves and the orbital branch of the trigeminal. The two 

 orbitosphenoids are often united on the floor of the skull so as to 

 form a median bone, which, when the cranial cavity is reduced, 

 may be represented by a cartilaginous or membranous septum. 



The roof of the skull is formed of bony plates, below which 

 remains of the primordial cartilaginous cranium are only rarely 

 retained. Close in front of the occipital are two parietal bones 

 (Par), and in front of these again the great frontal bone (Fr), on 

 each side of which is developed a post-frontal (FrP), which reaches 

 to the squamosal (pterotic), and takes part in the articulation with 

 the hyomandibular. 



In the ethmoid region there is in the prolongation of the base of 

 the cranium an unpaired cartilage or bone, the median or unpaired 

 ethmoid. This is covered ventrally by the large vomer, which is 

 attached to the parasphenoid. There are also two paired lateral 

 bones the lateral ethmoids or prsefrontals which are perforated by 

 the olfactory nerves and form the supports of the nasal pits 

 (nasal capsules). There are finally accessory membrane bones the 

 infra-orbital and supra-temporal which protect the cranial (sensory) 

 canals. 



A true maxillary apparatus appears for the first time in the 

 Selachians and Sturgeons, where a hyomandibular attached to the 

 auditory region serves to support the mandibular and hyoid arches 

 (figs. 571 H and 585 Hm). The upper part of the mandibular arch 

 (the palatoquadrate) is usually moveably attached to the skull by 

 ligaments. In the Teleostei the mandibular suspensorium is divided 

 into several parts, and the branchial operculum is attached to it. 

 The upper part is formed of a hyomandibular, and two bones called 

 by Cuvier the symplectic and tympanic (metapterygoid) ; the prse- 

 operculum forms the middle part, and finally the lower part, which 

 bears the articulation of the lower jaw, is formed by the quadrate or 

 quadrato-jugal. The flat osseous plates applied to the hinder edge 



