72 ALUS. [Vol. XVIII. 



oned where it fits in between the laminar processes, and being 

 capped with cartilage. The small remnant of the chondrocranium 

 that so lines the dorsal edge of the alisphenoid, thus presents the 

 same intimate but wholly independent relation to the purely der- 

 mal frontal, that the cartilage of the lateral edge of the chondro- 

 cranium, in Amia, does to the purely dermal squamosal of that 

 fish (No. 64, p. 188). 



The two arms of the Y-shaped ridge of the frontal, and the 

 corresponding portions of the related alisphenoid, enclose between 

 themselves and the corresponding parts of the opposite side of the 

 head a small anterior portion, or recess, of the cranial cavity. This 

 recess lodges the fore brain and the lobi olfactorii. Anteriorly and 

 ventrally it is not enclosed by bone, the bony brain case there being 

 open toward the orbit. This opening is the orbital opening of the 

 brain case, or orbital fontanelle of my descriptions of Amia (No. 

 5). Its edges give attachment, on each side of the head, to a mem- 

 brane, which runs mesially downward and forward and connects 

 with the hind end of the median, unpaired, membranous inter- 

 orbital septum. The two membranes, one on each side of the 

 head, thus close the orbital opening of the brain case, and form the 

 floor of the fore-brain recess. That part of the hind edge of the 

 median, membranous interorbital septum that is joined by, or gives 

 attachment to, the two lateral membranes, is much thickened, 

 especially in its upper and middle portions, and in the latter por- 

 tion contains a nodule of tough fibrous or fibro-cartilaginous tis- 

 sue. From this nodule the tough, fibrous, or fibro-cartilaginous 

 eyestalks of the fish take their origin. 



The antero-dorsal part of the orbital opening of the brain case 

 is small, lies between the anterior arms of the Y-shaped processes 

 of the frontals of the two sides of the head, and transmits the 

 olfactory nerves. These nerves, after leaving the brain case, run 

 forward in a median anterior extension of the cranial cavity 

 which lies between the ventral surface of the frontals and the 

 spreading, dorsal edge of the membranous interorbital septum. 

 At about the anterior quarter of the septum the cavity ends and 

 the olfactory nerve, on each side, there pierces the septum, later- 

 ally, and enters the orbit. 



