PLATANISTA. 509 



plates as thin as tissue paper directed forwards and upwards, and enclosing a wide 

 orifice that is downwardly and backwardly continuous with the posterior opening 

 iato the pterygoid sinus. These two plates are wedged in between the frontal, 

 maxillary and palatal ; the greater portion of the internal surface of the inner 

 plate being applied to the palatal ; its free, anterior margin forms the upper 

 boundary of the large round infra-orbital canal of the maxillary and part of the 

 roof of the pterygomaxillary fissure, and constitutes its posterior wall. The 

 outer plate is opposed to the maxillary surface of the temporal plate of the frontal, 

 where it forms the anterior superior boundary of the pterygomaxillary fissure. 

 The alisphenoidal and orbitosphenoidal portions of the external plate project inwards 

 and downwards over the nasal surface of the internal plate, which forms the greater 

 part of the outer wall of the lower two-thirds of the nasal canals. Erom the 

 anterior point of union of the internal and external plates, a long outwardly curved 

 narrow thin spictilar plate {op., figs. 13 and 14, PI. XL), grooved on its inner 

 surface, is projected forwards and appears in the iimer wall of the apex of the 

 orbital cavity. It is at first internal to and below the tract on the frontal traversed 

 by the nerves and vessels of the orbit, and afterwards forwards and outwards to 

 assist with the maxillary in forming its inner and upper waU. In adult skuUs, this 

 process appears as a fragile spiculum in the upper wall of the orbit between the 

 frontal and maxillary. 



The posterior half of the internal plate of the pterygoid, anterior to its nasal 

 surface, is applied to the outer surface of the lower portion of the palatine and by 

 the anterior half to the sides of the vomer, and to the vertical plate of the maxillary. 

 The anterior opening between it and the external plate is opposed to the posterior 

 orifice of the sinus between the two infra- orbital plates of the maxilla, and is con- 

 tinuous with the imperfection of ossification that exists on the anterior surface of 

 the maxilla, below the infra-orbital foramina. The pterygoid completely excludes 

 the palatine from the palate, as first pointed out by Eschricht. 



Parietal (PI. XL, figs. 6 and 7). — The parietal when removed from the 

 skull is more or less quadrangular, but in position it is separated above between the 

 exoccipital, supra-occipital and frontal ; narrowing to a point between the squamosal 

 and the frontal, where its lower extremity is in contact with the pterygoid, which is 

 applied over it. Its anterior beveUed border is applied over the posterior border of 

 the temporal ; the upper extremity of the margin of the bone presenting a thick- 

 ened portion where it is wedged in between the external angle of the supra-occipital 

 margin of the frontal and the supra-occipital. I have not been able to detect any 

 interparietal. The supra-occipital border is directed downwards and backwards. 

 In youth this margin simply overlaps the supra-occipital bending inwards, but in 

 the adult skull it becomes much intensified and is bent outwards. The 

 ex-occipital surface is short and overlaps the exoccipital, and anterior to it is the 

 rough surface to which the squamosal is applied. The outer sm-face of the 

 antero-inferior angle of the bone presents a smooth pit and a small sharp edge 

 for articulation with the upper thickened border of the anterior division of the wings of 



