PLATANISTA. 505 



projecting pterygoid process of the squamous which lies on the pterygoid ; next, 

 the pterygoid itself ; and lastly, the anterior haK of the alisphenoid. The anterior 

 surface of the body has not united with the presphenoid in the specimen 14 inches 

 long, and even in adult skulls this suture is always more or less distinct. The 

 anterior portion of the under surface of the body is rod-like in the middle to 

 permit of its fitting into the deep groove of the vomer, while the sides slope upwards 

 and outwards and rest in the deep grooves on the posterior sphenoidal processes of 

 the pterygoid, which continue forwards from the depending tympanic plates to the 

 external margins of the internal nares ; these plates thus extend from the exocci- 

 pital on to a line with the posterior margin of the vomer. The basisphenoid portion 

 contributed to these plates is thus comparatively small, not extending forwards 

 "beyond the two posterior thirds of the bone. They are directed downwards, back- 

 wards and outwards, and although they become firmly amalgamated with the same 

 processes of the basi-occipital, they are separate in the adult skull from the similar 

 plates of the pterygoid which overlap them. 



The posterior joortion of the alisphenoid rests, as already stated, on the rough 

 surface of the basisphenoid external to the pituitary fossa, and is perforated near 

 its base by what appears to be the foramen spinosum. Its upper external angle 

 has a marked depression on which the Gasserian ganglion lies. In young skulls 

 this portion of the wing is very thin, and its outward angle abuts against the 

 anterior margin of the internal auditory [meatus ; the periotic in youth projecting 

 into the lateral wall of the skull, while the tympanic below it forms part of its 

 floor. In adult life, this portion of the alisphenoid is enormously thickened and 

 completely consolidated with the underlying process of the basi-occipital ; the two 

 simulating a periotic in form and consistence and in their position in supporting the 

 Gasserian ganglion. By their after outward growth and by the inward thickening 

 of the base of the parietal against which they abut, the true periotic and tym- 

 panic are entirely excluded from the inner wall of the cranium ; and the imperfec- 

 tion at the base of the skuU in this region, which in the young is nearly continuous 

 from before backwards, is divided into two distinct sections, the foramen lacerum 

 medium and the foramen lacerum posterius. 



The upper margin of the posterior end of the body of the bone, although it 

 does not form any part of the pituitary fossa, is so close to its posterior margin that 

 it may almost be regarded as part of that border. 



The orbitospJienoid. — This bone, which is quite distinct from the basisphenoid 

 in youth and evidently for many years afterwards, unites apparently with the 

 mesethmoid in uterine life. In the young, however, and even in the adult, the 

 anterior limits of the bone in the interior of the skull can be detected, but its posi- 

 tion undergoes a remarkable change from youth to age. In the former it constitutes 

 part of the floor of the cranium, being in the same axis as the basisphenoid, but in 

 the latter stage it is almost at right angles to that bone and forms the lower portion 

 of the anterior wall of the cranial cavity. During these changes, what appears to be 

 the anterior limit of the bone becomes more intensified than it is in youth, assuming 



Q 3 



