SKULL 875 



(sometimes the Mastoid of Parker), which is often very small and 

 only develops several irregular ossifications that soon fuse with the 

 supra-occipital. The Opisthotic {Mastoid of Selenka) lies between 

 the Epiotic and the Lateral Occipitals, with which last it ultimately 

 fuses, and in some birds — Larus (Gull) for example — it actually 

 helps to bound the foramen magmim. 



The Basisphenoids are ventrally overlaid by a pair of membrane- 

 bones, the Basitemporals of some authors, and laterally they help 

 to form the lower and anterior border of the auditory meatus 

 (Ear, p. 176), while each Eustachian tube (/. c.) is, roughly speaking, 

 accompanied and partly covered by a canal for the passage of the 

 Carotids (p. 76) and some of their branches. The Basisphenoids 

 frequently articulate with the Pterygoids by the basipterygoid 

 PROCESSES (p. 28), and their dorsal surface supports the greater 

 portion of the base of the Brain, forming with the adjoining end of 

 Preesphenoids the sella turcica (p. 52). Dorsi- laterally the Basi- 

 sphenoid is joined by the Alisphenoid, which takes the greater 

 share in forming the posterior wall of the orbit. Between the 

 Alisphenoid and the Prootic the 2nd and 3rd branches of the nerims 

 trigeminus issue through one or two foramina, and laterally the 

 Alisphenoid joins the Squamosal, which in most cases separates it 

 from contact with the Parietal, while dorsally it meets the Frontal. 

 Forwards the Basisphenoid and Alisphenoid are continued into the 

 Prsesphenoid and Orbitosphenoid respectively, and these last, except 

 at their posterior end, are practically unpaired — the right and left 

 half being, so to say, pressed together by the extraordinarily 

 developed eyeballs into a median interorbital septum, which is 

 dorsally overlaid by the Frontals, while it is continued forwards 

 as the internasal septum ; but complications are produced by the 

 development of lateral outgrowths, which as Turbinals or nasal 

 conchse serve partly to enlarge the surface and partly as protective 

 chambers of the olfactory and nasal mucous membranes. There 

 is often no sharp line of demarcation between the Orbitosphenoid 

 and Prsesphenoid, because the extent to which the cartilaginous 

 septum ossifies is subject to individual variation;^ but the forame^i 

 opticum for the passage of the optic nerve (Eye, p. 233) always lies 

 near the base of the Orbitosphenoid. The junction of this last 

 named with the Alisphenoid is also marked by a variable number 



^ An incomplete internasal septum produces what are known as nares pervias, 

 characteristic of Colyrtihxis, Podicipes, Phaethon (alone among the Steganopodes), 

 Ardeidm (excl. Baleeniceps and Cancroma) ScojJus, Ciconia, Phcenicopterus, 

 Anseres, Palamedea, CathartidsR, Rallidse, Gruidee, Cariama, Otis, Eurypyga, 

 Podica, not in Ehinochetus, Limicolas, LaridsB, Alcidas, and various Passeres. 

 When the septum is complete these are of course Jiares impcrviaz, no matter 

 whether it remains cartilaginous or ossifies more or less, as in many Striges, 

 Podargus, Steato7-nis and Trogonidae. 



