No. i.] MORPHOLOGY OF THE PETROSAL BONE. 13 







portion of the membrane of each side closes, in such cases, the 

 corresponding half of the orbital fohtanelle. Where there is an 

 interorbital septum and no teleostean basisphenoid bone, as in 

 the Characinidae and Cyprinidae, the membranes of opposite 

 sides unite in their mid-ventral portion and are continued back- 

 ward to form the membranous basisphenoid and the pituitary 

 fossa, forming at the same time the floor of the cranial cavity 

 and the roof of the eye-muscle canal. The orbital opening of 

 the latter canal thus lies external to, and inferior to, this part 

 of the membrane. This is all evident in itself from Brooks' 

 and Sagemehl's several descriptions and statements, and is 

 practically shown to be the case in one of Vrolik's figures of 

 Esox lucius (No. 32, Fig. 9). 



Through the orbital fontanelle, as above defined, the optic 

 nerves are said by Sagemehl to issue, in all the teleosts described 

 by him (No. 27, p. 70, and No. 28, p. 570). The nervus 

 oculomotorius on each side is said by him to issue through the 

 same opening, or through a special foramen in that part of the 

 petrosal that lies lateral to, and in front of, the pituitary fossa. 

 The trochlearis issues through the fontanelle in Cobitis and its 

 related species, but in all the other fishes described it pierces 

 the alisphenoid by a small foramen, the exact position of which 

 is not given. 



Each half of the orbital fontanelle in certain of these fishes 

 thus corresponds exactly, in the structures it transmits, the 

 optic nerve alone excepted, to the dorsal part of the tall orbital 

 opening of the eye-muscle canal of Amia. In the other fishes 

 described, it differs only in that the trochlearis and oculomoto- 

 rius have become enclosed in the edges of the bones that bound 

 the opening. In none of the fishes described is there an upper 

 lateral chamber of the eye-muscle canal as in Amia. 



The arrangement and disposition of the parts here under con- 

 sideration thus indicate that that part of the internal fibrous 

 layer of the external limiting membrane of the dura mater of 

 Sagemehl's definitions that forms on each side of the head of 

 Amia the median wall of the upper lateral chamber of the eye- 

 muscle canal has, in all the teleosts described by Sagemehl, 

 fused more or less completely with that subjacent, external, 



