THE PIGEON. 191 



40. In the young bird the antero-ventral region of each 

 side of the brain-case is formed by a separate bone, the 

 squamosal (sg), bounded by the frontal and parietal above 

 and behind, by the alisphenoid (§ 42) in front, and by the 

 exoccipital below. 



41. At the posterior margin of each orbit the curved side- 

 walls of the skull turn sharply inwards, forming a thin plate 

 of bone — the orbital plate — which constitutes at once the 

 anterior wall of the brain-case and the posterior wall of the 

 orbit. 



42. In the young bird the orbital plate consists of two 

 parts, — an upper and smaller portion, the orbital plate of 

 the frontal (or.fr), an inturned portion of the latter 

 bone, and a lower and larger portion, forming a distinct 

 bone, the alisphenoid {al.s). 



43. The interorbital septum {i.o.s), a thin vertical plate 

 of bone separating the orbits from one another, and uniting 

 posteriorly with the orbital plates. 



44. The anterior part of its interorbital septum consists, 

 in the young bird, of a separate bone, the mesethmoid 

 {m.eth), the posterior part representing the presphenoid. 



45. In the dried skull the orbits are placed in com- 

 munication with one another by two large vacuities, or 

 interorbital fenestrse, situated one above the other, and 

 occurring partly in the orbital plates, partly in the inter- 

 orbital septum : in the recent state they are filled in 

 by membrane. 



46. In the membrane filling the uppermost interorbital 

 fenestra there is a median aperture, the olfactory fora- 

 men, for the transmission of the olfactory nerve from the 

 brain : this is continued on each side by a groove, visible in 

 the dried skull, which runs along the dorsal edge of the 

 interorbital septum and, at its anterior end, passes into an 



