The centre of the foremost part of the skull is occupied by the _ethmoid_ (3), which shows great variations as regards its extent and the degree of ossification; it may extend backwards into the interorbital septum, and reach the orbitosphenoids, or may be confined to the extremity of the skull; it may remain entirely cartilaginous, or ossify into a lamina which separates the two orbits and encloses an anterior prolongation of the brain-case, along which the olfactory nerves pass: modifications occurring again in higher vertebrates. A paired ossification attached to the fore-part of the ethmoid is the _pre-frontals_ (2), which form the base of the nasal fossa. 2. _Membrane-bones attached to the primordial skull_.--To this group belong the _parietals_ (7) and _frontals_ (1). The _squamosal_ (12) has been mentioned above in connection with the mastoid. The _supraorbital_ is always small, and frequently absent. The lower surface of the skull is protected by the _basisphenoid_ (parasphenoid) (6) and the _vomer_ (16), both of which, especially the latter, may be armed with teeth.[9] 3. _Cartilage bones of the alimentary portion of the visceral skeleton of the skull_.--The suspensorium consists of three cartilage-bones, and affords a base for the opercular apparatus as well as a point of attachment to the hyoid, whilst in front it is connected with the palato-pterygo-palatine arch. They are the _hyomandibular_ (23), _symplectic_ (31), and _quadrate_ (26), connected by means of the _metapterygoid_ (27) with the _ecto-_ (24) and _ento-pterygoid_ (25), the foremost bone of the arch being the _palatine_ (22). All these bones have been sufficiently described above (p. 55), and it remains only to be mentioned that the bones of the palatine arch are but rarely absent, as for instance in _Murænophis_; and that the symplectic does not extend to the articulary of the mandible, as in _Amia_ and _Lepidosteus_, though its suspensory relation to the Meckelian cartilage is still indicated by a ligament which connects the two pieces. Of the mandibulary bones the _articulary_ (35) is distinctly part of Meckel’s cartilage. Frequently another portion of cartilage below the articulary remains persistent, or is replaced by a separate membrane-bone, the angular.