No. i.] MORPHOLOGY OF THE PETROSAL BONE. 3 



morphologically in the adult also, in front of the foramen 

 by which the olfactory nerve leaves the cranial cavity. The 

 optic fenestra is, according to Sagemehl, simply the greatly 

 enlarged foramen of the nervus opticus, but it includes, in 

 its posterior portion, as will later be shown, the tall orbital 

 opening of my descriptions of the eye-muscle canal (No. 2), 

 which opening transmits the oculomotorius, trochlearis, pro- 

 fundus, and abducens nerves, and is the fused foramina of 

 those nerves and that of the_vena ophthalmica (No. 2). 



The orbitosphenoid and alisphenoid, both of which form 

 parts of the side wall of the skull, thus lie respectively 

 between the olfactory and profundus nerves, and the profundus' 

 and trigeminus. Posterior to the alisphenoid the petrosal 

 occupies a similar position between the facialis and glosso- 

 pharyngeus. 



The profundus, trigeminus, facialis, and glossopharyngeus 

 are all generally considered as segmental nerves; the olfacto- 

 rius is sometimes so considered (Nos. 6, 18); and Marshall 

 suggests (No. 1 8, p. 38) that the opticus also may possibly be 

 of segmental value. 



The orbitosphenoid, alisphenoid, and petrosal bones of 

 Amia thus have, in general position, the same relation to seg- 

 mental nerves that the several components of the occipitale 

 laterale have (No. 2). This markedly interneural, and hence 

 segmental or intersegmental, whichever it may be, position of 

 the bones in Amia, thus gives no support whatever to Vrolik's 

 conclusion (No. 32, pp. 240, 251) that the occipitale laterale 

 and petrosal of fishes are ossifications of the side wall of the 

 brain case, formed, respectively, around the vagus and facial 

 foramina. The alisphenoid would naturally, under Vrolik's 

 interpretation of the bones, be an ossification formed around 

 the trigeminal foramina. 



The postfrontal of Sagemehl, my postorbital ossification, 

 forms part of the dorso-lateral edge of the skull in the region 

 between the trigeminus and facialis. 



The two so-called halves of the basisphenoid lie in the base 

 of the skull in the region between the olfactorius and profun- 

 dus, if not perhaps between the opticus and profundus. 



