— 162 — 



brain case. The latter surface inclines slightly forward, but lies, as just above stated, in a nearly 

 transverse position. It is formed, as usual, by the proötic, alisphenoid and sphenotic, but the latter 

 bone is reduced and forms only the small dorso-lateral corner of the wall. The wide roof of the orbit 

 is formed mainly by the frontal but partly also by the ectethmoid and sphenotic. Its floor is formed 

 in part by the expanded base of the pedicle of the ectethmoid, in part by the wide parasphenoid, 

 and in part also by the large orbital shelf of the second bone of the infraorbital series. 



The interorbital wall is relatively thick. It is single in its ventral but double in its dorsal 

 portion, this latter portion enclosing an anterior Prolongation of the cranial cavity, which extends 

 the füll length of the interorbital region and lodges the olfactory nerves. The anterior half, approx- 

 imately, of the wall is cartilaginous, and here the olfactory prolongation of the cranial cavity is roofed 

 by a wide flat band of cartilage, already referred to when describing the rostral Chamber. The 

 posterior half of the wall is formed, in its ventral portion, by a median interorbital process of the 

 parasphenoid, and in its dorsal portion by the alisphenoid of either side, the expanded dorsal edges 

 of the latter bones not quite touching in the median line and so leaving a narrow longitudinal opening 

 in the roof of this part of the olfactory extension of the cranial cavity. A ventral f lange to the frontal, 

 found more or less developed in all the other fishes of the group is here wholly wanting. 



The cartilaginous portion of the interorbital wall is perforated, close to its antero-dorsal corner, 

 by a large oval opening which leads from orbit to orbit and is closed, in the recent state, by membrane. 

 This membrane is single in its ventral but double in its dorsal portion, the latter portion enclosing 

 the anterior end of the olfactory prolongation of the cranial cavity, which prolongation extends 

 to the hind surface of the short pillar of cartilage that forms the hind wall of the rostral chamber. 

 The membrane is pierced, on either side, by the olfactory nerve, that nerve then traversing the 

 extreme anterior end of the orbit to enter and traverse the opening that leads from the orbit into 

 the nasal pit, that pit being, as already stated, confluent with the rostral chamber. From the single, 

 ventral portion of the membrane, ventral to the olfactory nerves, theobliqui muscles have their origins. 



The median interorbital process of the parasphenoid, above referred to, is a tall broad Y- 

 shaped process, the spreading arms of which may be said to present three regions. In the anterior 

 region each arm is formed by a thin plate of bone which overlaps externally the anterior edge of 

 the corresponding alisphenoid, and, anterior to that bone, lies against the external surface of a part 

 of the cartilage that encloses the interorbital extension of the cranial cavity. In the middle region, 

 the dorsal edge of either arm is thickened and suturates with the ventral edge of the corresponding 

 alisphenoid. In this region the dorsal surface of the process seems to form the floor of the interorbital 

 extension of the cranial cavity, but there may here have been delicate lining plates of cartilage that 

 were lost in dissection. The basal portions of the arms of the Y are connected, at the hind edge of 

 this middle region, by a delicate transverse web of bone. In the posterior region, the arms of the Y 

 are short and spread considerably, thus forming, on the dorsal edge of this part of the process, a 

 basin-like depression in which the optic chiasma rests. The alisphenoid of either side arches above 

 this optic depression of the process of the parasphenoid and, anterior to it, suturates, as already 

 stated, with the corresponding dorsal edge of the olfactory portion of the process. The posterior 

 corner of each arm of the Y is prolonged and terminates in a point directed toward, or even reaching 

 the dorsal edge of the orbital surface of the proötic. A large fenestration of the interorbital wall is 

 thus here formed which may be wholly enclosed by the bounding bones, those bones being the ali- 

 sphenoids and proötics of either side and the median interorbital process of the parasphenoid. The 



