PLATANISTA. 503 



portion on its anterior fourth. This thickening becomes much intensified with age 

 and deeply concave on its posterior aspect. The tympanic borders also become 

 strongly developed as the animal increases in years, and it appears evident that, as 

 the basisphenoid and the anterior thickened portion of the basi-occipital constitute 

 the greater part of the upper wall of the posterior sinus, the increased thick- 

 ness of this portion of the basicranial axis of the skull is for the protection of the 

 brain against the pressure which it would sustain in the respiratory effort were its 

 structure in this portion of a less solid nature. 



The posterior three-fourths of the bone is very much thinner than in its ante- 

 rior fourth. In the very young example captured in August, the bone is rather 

 deeply concave in the mesial line for the reception of the medulla oblongata, with 

 a concave depression on its sides for the reception of the lobes of the cerebellum. 

 The concavity for the medulla is marked by two posteriorly divergent grooves, 

 probably arterial. On the anterior extremity of this surface there is a small, 

 well-marked depression, apparently for the reception of the pituitary body. In 

 the foetal skull the pituitary body is true to its position on the hinder end 

 of the basisphenoid, as is demonstrated by the fact that a pin passed through 

 the middle of the cartilage separating the two bones appears on the inner surface 

 of the base of the skull posterior to the pituitary fossa. The remarkable change 

 of position which overtakes this fossa is, however, inaugurated at a very early 

 period. Passing outwards and backwards from the sides of the pituitary depression 

 there is a faint ridge wliich terminates in a strong process of bone, which is 

 transverse to the basicranial axis and defines on either side the anterior limit of the 

 cerebellar lobe. This transverse process is separated by a transverse incision from 

 another but less prominent process lying immediately below it. These two pro- 

 cesses contribute to define the anterior border of the foramen lacerum posterius. 

 With increasing age the uppermost process is prolonged inwards to near the 

 median line of the skull, constituting two false, posterior clinoid processes, 

 defining the hinder border of the pituitary fossa. In some skulls I have observed 

 only one on the right side, which is the process usually most strongly marked. 

 The anterior surface of the base of this process to the free margin of the basi-occi- 

 pital external to the pituitary fossa, presents a rough triangular surface on which 

 lies the posterior angle of the alisphenoid. With age these two united portions of 

 the two basicranial bones become enormously tliickened, projecting outwards and 

 slightly backwards. 



At the anterior angle of the exoccipital surface, there are two processes, a 

 superior and inferior, the former contributing to form the anterior surface of the 

 base of the process that defines the front border of the anterior condyloid foramen, 

 the latter being the hinder termination of the downwardly and outwardly pro- 

 jecting plate against which the tympanic abuts. This plate terminates anteriorly 

 in a process for the support of the basisphenoid, being applied to the inner 

 surface of the base of the corresponding plate of that bone. The anterior surface 

 presents a transversely elongated concave rough surface, which becomes united to 



