78 ALUS. [Vol. XVIII. 



forward, upward and but slightly laterally, at an angle of about 

 45° to a horizontal plane. The dorsal end of its antero-mesial 

 edge is thickened and slightly notched with a V-shaped notch, thus 

 giving to the dorsal edge of the bone somewhat the shape of a 

 Y with a long shank and very short arms. The dorsal end of the 

 postero-lateral edge of the bone is bent slightly backward, and 

 lies in a nearly vertical position. The entire dorsal edge of the 

 bone is edged with cartilage and is slightly tenonned its full length, 

 both on its external and internal surfaces. This tenoned end of 

 the bone, with its cap of cartilage, fits into the Y-shaped space be- 

 tween the three laminar processes that form the Y-shaped ridge 

 on the ventral surface of the frontal. 



Laterally the alisphenoid abuts against the postorbital ossifica- 

 tion, and ventrally against the petrosal and basisphenoid. Antero- 

 mesially it has a free edge which presents a large, shallow, reen- 

 trant angle. This edge of the bone forms the ventral and larger 

 part of the lateral boundary of the large, median, orbital opening 

 of the brain case. The dorsal part of the lateral edge of this open- 

 ing, and part of its dorsal edge also, are formed, on each side, as 

 already stated, by the ventral edge of the anterior part of the ante- 

 rior arm of the Y-shaped ridge of the frontal. Its ventral edge 

 is formed by the anterior edge of the median basisphenoid. The 

 dorsal and smaller part of the opening lies immediately ventral 

 to the lobi olfactorii ; its larger, ventral portion lying immediately 

 ventral to the fore brain and the optic chiasma. The opening is, 

 in the recent state, entirely closed by membrane excepting in its 

 extreme dorsal portion, where it transmits the olfactory nerves, 

 as already described. Through its larger, ventral portion it trans- 

 mits, on each side, the nervi opticus and trochlearis, both of which 

 pierce the membrane that closes the opening and issue directly 

 from the cranial cavity into the hind end of the orbit. The opticus 

 pierces the membrane close to the middle line of the head, between 

 the anterior edge of the basisphenoid and the hind end of the 

 tough fibrous nodule that gives origin to the eye-stalks. It lies, 

 as it issues, postero-ventral to the eye-stalk, slightly in front of 

 the extreme hind end of the orbit and about midway in height 

 between the floor and roof of the orbit. It issues through the 

 extreme hind edge of the interorbital septum as much as, or even 



