506 CETACEA. 



the character of a pronounced sutural line. The bone is referable to two portions, 

 a body and wings ; the latter extend quite as far on to the side of the skull as do 

 the alisphenoids. The wing-like portion presents two divisions, a posterior and 

 anterior ; the former constituting the front border of the sphenoidal fissure in the 

 cavity of the skull, and the latter, above it, projecting into the base of the posterior 

 angle of the frontal. There is apparently no mesethmoid spine, as that portion of 

 the mesethmoid is let into the anterior end of the body of the orbitosphenoid. The 

 ridge of the falx-cerebri can be traced on to the middle of the body in some young 

 skulls, whereas in adults the body is either concave or pitted irregularly. Half- 

 way between the extremity of the anterior division of the wing and the body of 

 the bone, in what appears to be its mesethmoid border, there is usually, in young 

 skulls, a considerable foramen for the passage of blood vessels ; this may be either 

 round or transversely elongated, but in adult skulls it becomes almost entirely 

 obhterated. 



The most remarkable feature, however, of the orbitosphenoid is the circumstance 

 that it is traversed halfway between its anterior and posterior borders, and about 

 halfway between its mesial line and the outer extremity of its wings, by an 

 extremely small optic foramen ; this is so minute and so like to the sentient fora- 

 mina that occur scattered over the inner wall of the skull, that it is very apt 

 to be passed over as one of these. Each optic foramen (PL XL, fig. 1, op) 

 is overhung by a fine bony plate, and a groove passes inwards and backwards 

 in young skulls, but in adiilt crania it is all but obliterated. In youth the foramina 

 are much more symmetrical in their position than in advanced life, and they always 

 occur in the place I have assigned them ; but in the latter period they are generally 

 close to the anterior border of the basisphenoid and frequently at different elevations, 

 one being usually higher than the other ; in some skulls the left is higher than the 

 right, and in others the right is on the higher level. The foramina are so minute 

 that they only admit a fine bristle, and in adult skulls it is difficult to pass even this 

 through the whole length of the canal, wliich is of considerable extent. A hair that 

 I inserted through the optic foramen of one skull again entered the interior of the 

 skull by the sphenoidal fissure ; but the cause of this will be apparent when the 

 course of the optic canal through the frontal bone is considered. 



The posterior division of the wing of the orbitosphenoid articulates by its 

 extremity with the frontal anteriorly, and by its outer and under surface (which is 

 broadly and deeply grooved for the branches of the fifth nerve passing out by the 

 sphenoidal fissure) with the outer extremity of the alisphenoid; this is overlapped by 

 the pterygoid and by the under surface of the base of the wings with the internal 

 superior plate of the pterygoid ; the body of the bone being supported by the vomer 

 (consult PI. XL, fig. 1, &c.) 



Ilesethmoid.— It is impossible, even in young skulls, to define the exact extent 

 of this bone, as it unites in uterine life with the frontals (PL XL, fig. 1, me) ; but 

 what appear to be its Umits are best seen in the internal and nasal aspect of the 

 cranium. It forms the upper portion of the posterior wall of the external nares 



