PLATANISTA. 523 



the pterygoid, the only portion which is not so invested being the upper half of 

 the superior division of the external surface. The latter lies in a vacuity defined by 

 the orbito-sphenoidj frontal, maxillary, palatine, and pterygoid, and corresponds to 

 the spheno-maxillary fossa, because the sphenoidal fissure and optic foramen open 

 into it. Close to its anterior margin, behind the large maxillary foramen already 

 mentioned, there is a thin small outwardly-projecting plate, perforated on its upper 

 surface by a minute foramen. A bristle passed downwards and forwards through 

 this appears at the middle of the anterior border of its lower half ; but when the 

 pterygoid is in position, the tract in question is lengthened above and below by the 

 opposition of the pterygoid to the outer surface of the bone. The canal is thus 

 defined by the pterygoid, palatine and maxillary, and is the pterygo-palatine canal. 

 An imperfection of ossification exists in the nasal surface of the bone and leads 

 into the canal, doubtless transmitting a nerve to the mucous surface of the nasal 

 canal. Immediately above this foramen there is another which leads directly into 

 the nasal canal. The inner surface of the palatine, which is much more limited in 

 its extent than the external, is applied by its upper half to a narrow surface on the 

 hinder end of the maxilla below the frontal plate of that bone, fig. 17 n, and its lower 

 half to the outside of the vomerine crest of the maxilla. The centre is thin and 

 marked by numerous imperfections of ossification, while its nasal borders are thickened 

 and the external slightly expanded. A thin plate projects forwards from the upper 

 portion of the anterior border of the bone at right angles to its long axis and 

 corresponds to the anterior half of the concavely-truncated upper external border 

 which embraces the base of the frontal plate of the maxilla. The lower half of the 

 internal surface is smooth and slightly convex. 



The nasal aspect of its posterior surface is smooth, but porous, being covered 

 with numerous minute foramina. By far the greater portion of the upper half 

 looks inwards, but a small part enters into the anterior wall of the nasal canal, 

 while its lower half is directed inwards and backwards. In looking into the nasal 

 cavity from below upwards, it is seen articulating by its inner or anterior margin 

 with the side of the rough triangular expansion of the vomer, anterior to its nasal 

 septum, and above this through the whole of its nasal course with the nasal surface 

 of the posterior extremity of the maxilla. Its external or posterior border, on 

 the other hand, is observed to lie along the anterior margin of the nasal surface 

 of the pterygoids, at the upper end of which the external margin of the wing 

 of the vomer is wedged in between it and the pterygoid, the remainder of the 

 margin being applied to the former. Its upper margin is in contact with a 

 transverse ridge on the anterior surface of the frontal immediately external to 

 the mesethmoid, and its external angle articulates with that bone. In this 

 portion of its course it lies between the mesethmoid and vomer, the frontal, 

 maxillary, and pterygoid, and is above the level of the orbit, but far removed from 

 it posteriorly. 



3Ialar (PL XL., figs. 15 and 16). — This is a relatively small bone consisting, as 

 in Cetacea generally, of two portions, a round thick knob-like anterior, and a styli- 



