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 magnum. 
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 
(HAR, p. 176), while each Eustachian tube (/. ¢.) 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 BASIPrERYGOID 
PROCESSES (p. 28), and their dorsal surface supports the greater 
portion of the base of the Brain, forming with the adjoining end of 
Presphenoids 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 nervus 
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 
Presphenoid 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 
conche 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 Presphenoid, because the extent to which the cartilaginous 
septum ossifies is subject to individual variation ;+ but the foramen 
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 
1 An incomplete internasal septum produces what are known as nares perviz, 
characteristic of Colymbus, Podicipes, Phacthon (alone among the Steganopodes), 
Ardeidz (excl. Balwniceps and Cancroma) Scopus, Ciconia, Phenicopterus, 
Anseres, Palamedea, Cathartide, Rallide, Gruide, Cariama, Otis, Eurypyga, 
Podica, not in Rhinochetus, Limicole, Laride, Alcidx, and various Passeres. 
When the septum is complete these are of course nares impervie, no matter 
whether it remains cartilaginous or ossifies more or less, as in many Striges, 
Podargus, Steatornis and Trogonide. 
