504 CETACEA. 



the basisphenoid, and the posterior surface is crescentically concave before the 

 inferior border of the foramen magnum. The tympanic plate is thin rough and 

 slightly concave on its outer surface, and behind it is the elongated surface which 

 unites with the exoccipital. 



In the adult skull the line of union with the basisphenoid can sometimes be 

 . traced, but its union with the exoccipital cannot be detected. All the foregoing 

 processes become greatly intensified in the adult, more especially the process external 

 to the pituitary fossa; these project outwards beyond the cranial cavity as two 

 strong buttresses on which the parietals rest, although they do not unite with these 

 bones. Their extremities point to the internal auditory meatus, and their posterior 

 sides define anteriorly a strongly marked groove for the seventh pair of cranial nerves ; 

 immediately behind this there is another prominent groove for the eighth pair, 

 which arch over the strong process defining the anterior wall of the precondyloid 

 foramen and is so closely applied to the exoccipital that this foramen is all but 

 perfect. 



The basisphenoid (PL XL, fig. 11). — The alisphenoids are outwardly and up- 

 wardly curved, and their excephalic aspect much hollowed in the middle from before 

 backwards, but convex from side to side, with a considerable concavity at the base of 

 the wings for the reception of the lobes of the cerebrum. The wings may be referred 

 to two portions, an anterior, by far the higher and larger, and a posterior, backwardly 

 and upwardly projecting section, that lies on the rough surface of the basi-occipital 

 external to the pituitary fossa. The cerebral hollow of the anterior portion of the 

 wings is very thin and easily fractured, hence the imperfections in the figure (fig. 11), 

 but its upper extremity is very thick and rough. Immediately below this latter 

 portion there is a wide groove directed longitudinally forwards, leading to the sphe- 

 noidal fissure and prolonged backwards as a distinct process along which the two 

 superior divisions of the fifth nerve pass. There is a large crescentic foramen just 

 below the posterior process, a backward continuation of the groove for these divi- 

 sions of the fifth nerve and through which the inferior maxillary division of the 

 same nerve passes outwards through the alisphenoid and hence the foramen ovale. At 

 the base of the posterior division of the alisphenoid there is another foramen (spinosum) 

 which transmits a branch of the plexus at the base of the skull. The anterior divi- 

 sion of the alisphenoid is applied by its front margin to the orbitosphenoid, while by 

 its thickened superior rough surface it articulates with the inferior thickened rough 

 angle of the parietal ; it also rests on the external surface of the inferior posterior 

 external angle of the parietal, i.e., being external to that process with the frontal pro- 

 cess of the pterygoid external to it and excluding it from appearing in the outer wall 

 of the skull, although in the temporal fossa it is close behind the superior border of 

 the isolated portion of the pterygoid, that is wedged in between the frontal, parietal 

 and squamosal.^ At this portion, the wall of the cranium consists of three layers 

 of bone. These, from without inwards, are ; first, the downward and forwardly 



^ See Flower's remarks on the structure of the temporal fossa of Platanista as compared with that of 

 Iniay 1. c. p, 90. 



