510 CETACEA. 



the basisphenoid ; a process from that border being applied to the external surface 

 of the parietal. In youth, the petrous bone enters into the formation of the inner 

 wall of the cranium, and has thus the greater portion of the lower border of the 

 parietal applied to it. There is a small flattened process from the border of the 

 parietal resting on the petrous over the internal auditory foramen, always present 

 in young skulls. This process plays an important part in the economy of this region 

 of the skull, as it expands with increasing years, and materially contributes to the 

 after exclusion of the petrous from forming any portion of the inner wall of the 

 skull ; but in this it is much assisted by the growth of the false petrous process of 

 the basisphenoid, by the outward growth of the basi- occipital and by the inward 

 and anterior strengthening of the basicranial margins of the exoccipital. In adult 

 life, this portion of the parietal is closely applied to the postauditory process of 

 the basisphenoid, and anterior to that the border of the bone is bent downwards and 

 articulated to the posterior division of the wings of the basisphenoid ; forming the 

 inner and upper wall of that portion of the great ear-chamber, which is prolonged 

 forwards above the pterygoid process of the squamosal. By its inward growth it 

 also contributes to the division of this fissm^e in the cranial walls into two parts : 

 one the foramen lacerum anterius, defined anteriorly by the posterior division of the 

 wings of the basisphenoid, externally by the basisphenoid, posteriorly by the false 

 petrous portion of the basisphenoid by the shelving petrous processes of the parietal, 

 and superiorly by the thickened, lower border of the parietal ; the other the foramen 

 lacerum posterius, behind the two last-mentioned processes which define its 

 anterior border, its inner margin being formed by a deep concavity lying between the 

 Glasserian process of the basisphenoid and the strong, outward projection of the 

 basi-occipital which constitutes the anterior border of the precondyloid foramen of 

 the occipital. The posterior wall is formed exclusively by the extremity of the 

 downwardly, outwardly and forwardly projecting strong ridge of the exoccipital, 

 along wliich the lateral sinus is placed and which also defines the posterior wall of 

 the precondyloid foramen. Its upper wall is formed entirely by the thick, lower 

 border of the parietal. The inner wall of the bone is deeply concave, marked by 

 depressions for the cerebral convolutions and meningeal vessels, and by numerous 

 nutrient foramina. 



Frontal (PL XXXIX, figs. 1 and 2 /, and PL XL, fig. 1).— This bone 

 consists of a central, vertical or cerebro-nasal plate, the anterior surface of which, 

 with the exception of a narrow strip down the middle of its anterior aspect and 

 which forms the upper part of the hinder wall of the nares, is covered on either 

 side by the ascending plates of the maxillaries and partially by the upper extrem- 

 ities of the premaxillaries. Projecting outwards, forwards and downwards like a 

 pair of wings from the external borders of the vertical plate are the tempero-orbital 

 plates, to the inner surface of which the ascending plates of the maxillaries are 

 also applied. The orbit is placed on the under surface of the free extremity of the 

 plate and has behind it the pit for the reception of the zygoma. The posterior 

 surface of this plate constitutes the anterior wall of the temporal fossa. Project- 



