510 CETACEA. 



the basisplienoid ; 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 j)art 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 exoccij)ital. 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 fissiu'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 



do-miwardly, 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 waU of the bone is deeply concave, marked by 



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



nutrient foramina. 



Fronted (PI. XXXIX, figs. 1 and 2 /, and PI. XL, fig. 1).— This bone 

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

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

 wliich 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 mner surface of which the ascending plates of the maxillaries are 

 also applied. The orbit is placed on the under sui-face 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 waU of the temporal fossa. Project- 



