INIA GEOPFEBNSIS AND PONTOPOEIA BLAINVILLII. 93 



outer wall of the nasal passage, but not entering in the slightest degree into the com- 

 position of the free surface of the bony palate. 



In this disposition of the palatine and pterygoid bones Platanista stands alone 

 among Cetaceans ; even Inia presents no approximation to it. There are, however, in 

 the latter genus some peculiarities in this region by which it may be distinguished 

 from the ordinary Dolphins. 



Behind the posterior pair of teeth the palate loses its flatness, and begins to rise to a 

 ridge in the middle line and slope away at the sides towards the roof of the orbits. 

 The summit of the lidge is formed by the vomer, which is quite uncovered in the 

 middle line by the palate-bones. The inner edges of these bones, applied to the 

 surface of the vomer, are distinctly marked, and posteriorly are "4" distant from each 

 other. The suture between them and the maxillaries is completely obliterated, so that 

 theii' limits forwards and outwards cannot be definitely stated. As in the ordinary 

 Dolphins, the palatines have each an outstanding, nearly vertical, plate running outwards 

 and backwards, unattached posteriorly, and forming the upper part (in the natural 

 position of the skull) of the outer waU of the chamber which lodges the great post- 

 palatine air-sinus. This plate is slightly developed and very thin, perforated by 

 numerous large lacunse, and, owing to the non-development of the outer reflected portion 

 of the pterygoids, is completely fi'ee along its inferior edge. 



The pterygoids are comparatively simple, and also very thin and lacunated. As usual, 

 the upper or attached portion forms a ridge along the side of the cranium, continuous 

 posteriorly with the ridge on the side of the basisphenoid, which forms the inner 

 wall of the cavity for the lodgment of the ear-bones. This portion articulates by 

 nearly the whole of its inner edge with the hinder expanded part of the vomer, and 

 externally with the alisphenoid and orbitosphenoid. From its anterior part springs 

 the recurved descending plate which bounds externally the posterior nares, and, then 

 turning inwards and backwards, forms the anterior wall of these passages below the 

 palatines. This last-named plate of the pterygoid forms the hinder part of the bony 

 palate ; anteriorly it lies on the hinder free edge of the inferior surface of the vomer, 

 but does not quite cover it to the middle line ; behind the vomer it diverges rather 

 more from its fellow, leaving a gap of from -1" to -2" in breadth. Posteriorly each 

 terminates by a concave free margin. The third portion of the pterygoid, which 

 exists in all ordinary Dolphins (excluding the Physeteridce), and which when 

 present completely conceals that last described, being reflected from its hinder and 

 inner edge outwards and upwards to meet the edge of the projecting plate of the 

 palatine, and so close in the postpalatine sinus below, is wanting in Inia, or only 

 represented in the osseous cranium by some small irregular body-excrescences. The 

 result is that the cavity for the sinus is widely open below. It might be conjectured 

 that this plate, being thin, brittle, and much exposed to injury during the process of 

 cleanino- the skull, had been broken away. It is certainly possible that such is the case ; 



