Page:An Introduction to the Study of Fishes.djvu/117

Rh very large in Cyprinoids and Siluroids, in which they contribute to the formation of the side of the brain-case. The single Y-shaped Sphenoideum anterius (15) is as frequently absent as present; it forms the anterior margin of the fossa for the hypophysis. Finally, the post-frontal (4) belongs also to this group of cartilage-bones.

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 prefrontals (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.

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