SKELETON OF TELEOSTEI. 89 



very large in Cyprinoids and Siluroids, in wliicli tliey contri- 

 bute 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 back- 

 wards into the interorbital septum, and reach the orbito- 

 sphenoids, 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 verte- 

 brates. 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-hones attached to the primordial skull. — To 

 this group belong ih^parietals (7) Sindfrontals (1). The sqiia- 

 mosal (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 car- 

 tilage-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 



^ Stannius (pp. 60, 65) doubts the pure origin of these two bones from 

 membranous tissue, and is inclined to consider them as ' ' the extreme end of 

 the abortive axial system." 



