86 ALUS. [Vol. XVIII. 



On the outer surface of the petrosal, there is a depressed region, 

 narrow and shallow in front, but wide and deep behind. It forms 

 the anterior portion of the large depressed region, of which men- 

 tion has already been made, on the hind end of the lateral surface 

 of the skull. It lies opposite or immediately dorsal to the line 

 where, on the inner surface of the petrosal, the mesial, horizontal 

 process of the bone is given off. In the anterior half of the bone, 

 immediately dorsal to the bottom of this depression and immedi- 

 ately beneath the external surface of the bone, there is a short but 

 relatively large, nearly horizontal canal, or chamber (Fig. ii). 

 This chamber is always slightly curved, the hollow of the curve 

 presented upward, and it is wider at its middle point than at its 

 ends. The ventro-mesial portion of the chamber is separ- 

 ated from the dorso-lateral corner of the eye-muscle canal by only 

 a thin layer of bone. The external wall of the chamber is also 

 formed by a thin layer only of bone, which, at the anterior end of 

 the chamber, usually lies nearly flush .with the outer surface of the 

 petrosal, but at its posterior end rises in a gentle curve slightly 

 above the level of the adjacent portions of the bone. 



The anterior opening of this chamber is much larger than the 

 posterior one, and lies so close to the thick, anterior edge of the 

 petrosal that it can be considered as cut out of that edge, and as 

 opening onto it rather than onto the outer surface of the bone. It 

 is a long and relatively narrow opening, the long axis of which lies 

 in a nearly horizontal position. It extends forward and mesially 

 to the sutural line between the alisphenoid and petrosal, and there 

 lies directly lateral to the oculomotorius foramen, which foramen, 

 as already stated, lies between the basisphehoid and the basi- 

 sphenoidal process of the petrosal. The middle point of the open- 

 ing lies directly below the ventral end of the sutural line between 

 the alisphenoid and postorbital ossification ; and on the outer sur- 

 faces of the adjoining edges of those two bones there is a wide, 

 but very shallow, hour-glass-shaped depression, not always dis- 

 tinctly marked, which continues the line of the canal in the petrosal 

 upward and forward to the lower edge of the ventral, laminar 

 process of the frontal, where it disappears. The depression is 

 filled, in the recent state, with tough membranous tissue which is 

 continuous, anteriorly, with the membranes lining the orbit ; pos- 



