PLATANISTA. 507 



below the level of their upper border, the external margin of which is formed by 

 the premaxillse. The anterior surface is divided in two by the low vertical rido-e 

 of the mesethmoidal cartilage, which is prolonged forwards and slightly down- 

 wards as a laterally compressed plate, received below into the groove of the vomer. 

 In young skulls, the anterior surface of the mesethmoid is slightly concave, with 

 the ridge that traverses it incompletely ossified ; and it is perforated by two larger 

 foramina for blood vessels, one on either of its halves, with generally two smaller 

 foramina in its immediate neighbourhood. In adult life, the anterior surface has 

 the appearance as if the wings of the vomer were upwardly expanded, becoming 

 continuous superiorly with the upper portion of its vertical ridge ; but it is open 

 anteriorly for the downwardly thickened portion of the central plate, from which 

 the septal cartilage springs, and is continued forwards. The wings of the vomer 

 also appear, with age, to expand upwardly and outwardly over the foramina, and 

 almost to invest them. The vertical ridge is thus strongly prominent in adult life, 

 and the perforated sides of the bone are much more regular and concave than 

 in youth. 



On the internal surface of the skull, what appears to be the mesethmoid area 

 undergoes remarkable changes from youth to age. In the former period, the internal 

 frontal crest terminates in a vertical, rather long thick rounded ridge, which is 

 even prolonged in some skulls on to the body of the orbitosphenoid. At the lower 

 end of this ridge, there is a depression on either side of it, while immediately above, 

 there are three small foramina, of which one is larger and more prominent than the 

 others. In the adult skull, in which the orbitosphenoid and mesethmoidal areas are 

 completely vertical, the two feeble depressions of the young skull are converted into 

 deep pits, nearly half an inch in depth, separated from each other by a vertical plate, 

 situated rather deeply in the common opening to the two pits ; its upper half being 

 thick, whilst its lower half forms a thin vertical plate : the bases of the pits are 

 perforated by foramina. In the interior of these two pits we have thus the equivalent 

 of the ethmoidal spine and cribriform plate. In such skulls, the internal frontal 

 crest takes a curve to the right side, but, notwithstanding, the falx-cerebri is 

 attached to the upper thickened half of the vertical plate between the two pits. 

 On the lower extremity of the outer side of the mesethmoid anteriorly, the palatine 

 bone lies between it and the frontal, the mesethmoid plate forming the upper 

 posterior waU of the external nares. 



Pterygoids (PL XL, figs. 13 and 14). — The true nature of these bones was 

 first pointed out by Eschricht, ' in his " Memoir on Platanista " ; they form by far the 

 greater part of the palate. The anterior half of the external plate is convex and the 

 posterior half concave, especially near its hinder border. In the anterior two-thirds 

 of the palatal surface, the bones of either side, in the fully adult skull, are separ- 

 ated from each other by the interposed vertical plates of the maxillaries, but behind 

 this they are closely applied. The lateral concavity of their anterior halves confers 



» Trans. Roy. Acad. Soi., Copenhagen, ii, 1851. Translated by Wallioh, Ann. and Mag., Nat. Hist., 1852. 



