INIA GEOFFRENSIS AND PONTOPORIA 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 ridge 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 
their 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 wall of the chamber which lodges the great post- 
palatine air-sinus. This plate is slightly developed and very thin, perforated by 
numerous large lacune, and, owing to the non-development of the outer reflected portion 
of the pterygoids, is completely free 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 Physeteridw), 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 Jnia, or only 
represented in the osseous cranium by some small irregular body-excrescences. ‘he 
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 
cleaning the skull, had been broken away. It is certainly possible that such is the case; 
