508 CETACEA. 



a greater expansion on their palatal surfaces than occurs at their middle, which is 

 contracted by the downward and forward prolongation of the concavity that occurs 

 on their posterior halves. Behind the middle, the external plates are divergent, with 

 their posterior inferior angles rounded off into the nasal surfaces of their internal 

 plates ; the inferior margins of which are folded forwards in an involute manner. 

 Erom the central palatal contraction, backwards to the nasal plates, the two bones of 

 either side form a mesial long anteriorly pointed triangular palatal surface, consist- 

 ing of a fine network connecting the external and internal plates ; their anterior 

 extremities being also closed below by a similar structure. There are considerable 

 imperfections of ossification also in the nasal plates. The opening into the sinus of 

 the pterygoids exists internal and anterior to the posterior margin of the external 

 plate within the posterior nares, and external to its outer wall, where that is formed 

 by the internal plate. The orifice, in position, is oblong and is opposite and continu- 

 ous with the anterior termination of the longitudinal fissure, along which the Eusta- 

 chian tube runs backwards to the great cavity around the tympano-periotic. In 

 looking through the orifice, it is seen to tend directly forward into the cavity that 

 exists between the two plates of the maxilla ; a goose quill passed through it shows 

 that it is continuous with the large oval orifice and smaller foramina that exist on the 

 anterior surface of the maxilla, external to the premaxilla and below the level of 

 the external nares ; some of which foramina transmit the infra-orbital nerve. The 

 upper extremity of the anterior margin of the external plate is deeply notched, this 

 notch with the posterior extremity of the superior maxillary constituting the ptery- 

 gomaxillary fissure, which is continued below the sphenomaxillary fossa inter- 

 nally, and anteriorly terminating in three or four oval infra-orbital foramina. Erom 

 the upper border of the pterygomaxillary fissure, the ridge dividing the temporal 

 and zygomatic fossse is prolonged backwards to the foramen ovale. The superior 

 posterior half of the internal plate is applied over the alisphenoid, which is only 

 grooved externally by the inferior maxillary ; but a corresponding groove on the 

 former completes the canal in which the nerve lies. The perfect overlapping of the 

 alisphenoid by the pterygoid excludes the former from the outer wall of the 

 cranium. It articulates with the frontal, parietal and squamous ; the latter and its 

 downwardly projecting pterygotympanic process, on the other hand, so overlapping 

 this portion of the pterygoid that only a very small portion of it appears above the 

 foramen ovale internal and anterior to the base of the zygoma. The superior 

 anterior half of the internal plate of the pterygoid is also applied to the under 

 surface of the orbitosphenoid. It completes the sphenoidal fissure and confluent 

 foramen rotundum, and, by a thickened posteriorly concave sphenoidal process, over- 

 laps the free auditory plates of the basisphenoid. A thin plate internal and anterior 

 to this is applied to the under surface of the basisphenoid, its margin being wedged 

 in between the expanded base of the vomer and the orbitosphenoid, thus con- 

 stituting the roof and external wall of the internal nares. Erom the anterior 

 extremity of the orbitosphenoidal portion of the inner plate, at the posterior margin 

 of the pterygomaxillary fissure and immediately above the palatal, there are two 



