8o ALUS. [Vol. XVIII. 



Immediately behind the arms, or wing's, of the basisphenoid, 

 between them and the united anterior edges of the mesial, hori- 

 zontal processes of the petrosals, lies the relatively large, oval or 

 subtriangular opening of the pituitary fossa. Through this open- 

 ing the hypophysis cerebri and saccus vasculosus, enclosed in a 

 shallow sac of the dura mater, project downward into the anterior 

 end of the eye-muscle canal. 



The internal carotid arteries run upward immediately in front 

 of the front edge of the basisphenoid, as they do, in larvae of 

 Amia, in front of the dorso-anterior edge of the cartilaginous 

 transverse bar of the chondrocranium (No. 4, p. 497, and No. 5). 

 There, in Scomber, they pierce the membranes that close the 

 orbital opening of the brain case, close to the middle line of the 

 head, and enter the cranial cavity. 



The anterior edge of the shank of the basisphenoid gives attach- 

 ment to the hind edge of the ventral half, approximately, of the 

 membranous interorbital septum, the anterior edges of the wings 

 of the bone giving attachment, on each side, to the lower end of 

 the membrane that closes the orbital opening of the brain case. 



No Orbitosphenoid is found in Scomber. 



The Interorbital Septum is membranous throughout its en- 

 tire extent excepting only in those parts that are represented in 

 the shank of the basisphenoid, in the short dorsal and ventral 

 processes of the antorbital cartilage, and in the cartilaginous ridge 

 that connects those processes with each other along the middle 

 line of the posterior surface of the cartilage. Posterior to the 

 shank of the basisphenoid there is a median membranous septum 

 that extends backward from the shank of the bone to the hind end 

 of the eye-muscle canal, and it would seem to be, in certain speci- 

 mens and in part of its extent, a posterior extension of the inter- 

 orbital septum. 



The dorsal edge of the middle portion of the interorbital septum 

 encloses the proximal ends of the olfactory nerves. This part of 

 the septum thus belongs to the interorbital side walls of the skull 

 rather than to an unpaired interorbital septum, and the canal or 

 chamber that it encloses is, according to Sagemehl's descriptions, 

 an anterior part of the cranial cavity. Posterior to the hind end 

 of this canal, the dorsal, or dorso-posterior edge of the septum 



