No. i.] MORPHOLOGY OF THE PETROSAL BONE. 



cartilaginous, anterior clinoid wall." From the position of 

 these nodules Bridge was led to conclude that they were 

 probably ossifications that commenced in the strong fibrous 

 membranes that close the optic fenestrae of the fish, and that 

 they subsequently invaded the anterior clinoid wall. They 

 were homologized by him, under some reserve, with the " pre- 

 pituitary portion of the basisphenoid of other fishes"; but as 

 there is, in fishes, .no postpituitary portion of this bone, so far 

 as I can find described, the portion of the bone so specifically 

 defined by Bridge must have been intended to include the 

 entire bone. With this conclusion, as applied to the entire 

 bone of teleosts, Sagemehl entirely agrees, the principal 

 reason given by him being that if the bones are not the homo- 

 logues of the teleostean basisphenoid they would become 

 ossifications peculiar to Amia alone, among all fishes (No. 25, 

 p. 215). The disappearance of the wide cartilaginous bar that 

 separates the two bones in Amia, and their subsequent fusion 

 into the single impair bone of teleosts, is said by Sagemehl to 

 be brought about by the compressive action of the additional 

 recti muscles, which, in the latter fishes, find their way into 

 the eye-muscle canal. 



In my own earlier work on Amia I was led to strongly doubt 

 this homology of the so-called basisphenoids of that fish with 

 the teleostean basisphenoid. Later work, and a more special 

 study of the region, convince me that it is wholly wrong, as is 

 also the supposed homology of the teleostean basisphenoid 

 with the similarly named bone of higher vertebrates. 



In my earlier work (No. 2, pp. 406, 407) I found the 

 so-called basisphenoids of Amia of variable size and form, 

 not only in different specimens, but also on the two sides of 

 the head in the same specimen; and I found them developing 

 in connection with the origins of three of the recti muscles. I 

 considered them, as Bridge did, largely of membranous origin, 

 because they lay, in the adult, lateral to the internal carotid 

 arteries, while in larvae, those arteries, after having pierced 

 the basis cranii, ran upward and forward on each side of the 

 head, along the lateral surface of the ventral portion of 

 the cartilaginous, anterior clinoid bar. The further course 



