THE SKULL OF CUIM.ERA. 117 



in tlie region of the line of attachment of the membranous inter- 

 orbital wall, and directly ventro-lateral to the foramen by which 

 the nervus opticus traverses that membranous wall. 



The foramen opticum lies directly dorsal to a well-marked and 

 rounded transverse ridge on the floor of the ci-anial cavity, this 

 ridge marking the anterior end of the pituitary fossa and lience 

 being the presphenoid bolster of my descriptions of the Selachii. 

 The cranial cavity is here abruptly constricted, latero-mesially, 

 and this constriction, in its ventral portion, is produced by tlie 

 pinching in of the mesial membranous walls of the orbits, the 

 membrane of either orbit being pinched inward and downward 

 s.) that the internal surface of its ventral portion lies closely 

 upon the cartilage of the basis cranii. The membrane there 

 usually, but not always, becomes adherent to the cartilage, and 

 a second line of attachment of the membranous cranial wall, 

 lying mesial to the primitive line, is acquired. A depression is 

 thus formed in the ventral portion of the outer surface of the 

 membranous wall of each oi-bit, and it is tilled with tough fatty 

 connective tissue. The original line of attachment of the mem- 

 branous wall forms the ventro-lateral edge of this depression, 

 and lies latei-al to the foramen for the efferent pseudobranchial 

 artery, while the secondary line of attachment lies mesial to that 

 foramen, the membrane covering the foramen. Having issued 

 from its foramen, the efferent pseudobianchial artery runs pos- 

 teriorly beneath the covering membrane and issues in the cranial 

 cavity at the base of the posterior wall of the depression, there 

 lying posterior to the nervus opticus. It then immediately gives 

 off a recui'rent branch which perforates the posterior wall of the 

 depression, traverses the tough fatty tissue that fills the depres- 

 sion, and, joining the nervus opticus, accompanies that nerve to 

 the eyeball. After giving off this branch, the pseudobranchial 

 ai'tery turns mesially and immediately separates into anterior 

 and posterior cerebral arteries. 



When the secondary attachment of the membranous orbital 

 wall to the dorsal surface of the basis cranii is not strong, the 

 pseudobranchial foramen opens directly into the cranial cavity, 

 but when the attachment is strong it opens external to that 

 cavity but beneath the covering membrane. The nervus opticus, 

 after issuing from its foramen, lies directly vipon this adherent 

 membrane, and the membi'ane may there become thinned to 

 such an extent that, in dissections not carefully made, the 

 pseudobranchial foramen appears as a perforation of the sub- 

 orbital shelf lying wholly external to the interorbital wall ; this 

 apparently having been the condition in the specimen examined 

 in connection with my earlier work on the arteries of this fish 

 (AUis, 1912), where the foramen is said to open external to 

 the cranial wall. The foramen however lies, morphologically, in 

 the actual floor of the ci-anial cavity, as it is shown in Schauins- 

 land's figure of an embryo of Callorhynchrhs. In the Selachii 

 this foramen always lies, in all the specimens that I have 



