512 CETACEA. 



of the two branches are giyen off and pass along theinner aspect of the extremity 

 of the orbitofrontal plate to the orbit. Where the optic canal opens into the 

 sphenoidal canal the optic nerve is superior to the divisions of the fifth, which it 

 crosses to reach the orbit. In passing a bristle through the canal it is frequently 

 stopped in its progress by abutting against the outer wall of the sphenoidal canal 

 opposite its own exit from the orbitosphenoid, and it is this which sometimes 

 makes the bristle return into the skull by the sphenoidal fissure. In young, and 

 even in some adult, skulls the optic canal shows an imperfection of ossification 

 immediately above the pterygomaxillary fissure, its walls being thin and almost 

 transparent. 



The facial portion is the narrow elongated siirface on which the external basal 

 borders of the maxillary crests rest. It is directed downwards and forwards, narrow- 

 ing towards its upper extremity, which is placed over the median line of the tem- 

 poral fossa. The middle of its posterior border is marked by a deep notch, which is 

 best defined on the right side in adult skulls. 



The zygomatic process of the temporal (z, ^g. 2, PL XXXIX) is received 

 into a pit on the hinder margin of the lower end of the facial portion, imme- 

 diately behind or above the level of the superior border of the orbit, so that 

 there is no post-orbital process. In adult skulls, the inner surface of the anterior 

 end of the zygoma is sheathed by a thin plate of bone from the front wall of 

 the temporal fossa. Anterior to the zygomatic pit is the crescentic upper margin 

 of the orbit, which is a more or less triangular cavity, the apex of which is directed 

 inwards and backwards. It has a depth of about 1*50 inch in adult skuUs, with an 

 anterior diameter of not more than one inch. The inner wall is chiefly formed by 

 the thickened base of the malar, its roof being essentially frontal. Its floor is very 

 narrow and is formed exclusively by the backwardly and outwardly projecting 

 zygomatic process of the malar, which abuts against the zygomatic process of the 

 squamosal ; in this respect the orbit of this dolphin is more normal, so to speak, 

 than the orbit of such forms as Qlobicephalus, Orcella and their allies. The 

 malar vertical diameter of the orbit is little over half an inch. The malar floor 

 does not project so far forwards as the upper frontal border of the orbit, but the 

 deficiency is made up in life by a strong fibrous tissue which invests the eyebaU and 

 is external to the malar and to the frontal margin, so that the minute globe is almost 

 external to its orbit. 



The temporal, which is continuous with the orbitotemporal plate of the frontal, 

 but directed downwards, backwards and outwards, is triangular and received between 

 the parietal and pterygoids. 



Squamosal (PI. XXXIX, figs. 1 and 2, and PL XL, figs. 8 and 9).— The 

 distinguishing features of this bone are the great size and strength of its zygomatic 

 arch3 the remarkable postglenoid fossa and the strong downwardly and inwardly 

 projecting process which embraces the petrous and is applied to the outer surface 

 of the upper posterior extremity of the pterygoid and abuts against the anterior 

 border of the tympanic. 



