in.] THE CRANIAL SKELETON. 133 



bony cheek may, in man's own class, offer defects of ossifica- 

 tion, as in the Hare, or enormous prominences enclosing a 

 chamber, as in the Paca, or great swollen tuberosities, as in 

 the Mandrill. 



The anterior nares may be so high up as to approach the 

 summit of the skull, as we see in Cetacea. They may 

 present a very large aperture, as in the Tapir. They may be 

 widely separated one from the other in the middle line, as is 

 the case in mosj animals below Mammals, but not in all, as 

 we see in Chelonians and Crocodiles, which have a median, 

 single nasal opening, as in man's own class. 



As regards the lateral regions of the skull of man : 



(1) The temporal part may be roofed over and hidden from 

 view by plate-like processes extending out from the adjacent 

 bones, and which, meeting, enclose the temporal fossa and 

 muscle, giving the skull a fictitious appearance of great ca- 

 pacity. This is the case in the Turtle, the African Rodent 

 Lophiomys, and in the Frog Pelobates. 



(2) The mastoid part can hardly with propriety be so 

 called in lower forms, where the mastoid process becomes 

 much smaller or aborts altogether. 



The meatus auditorius externus may in the adults of very 

 high animals (e.g. the American Apes) be replaced by a wide, 

 nearly circular opening, as in the human skull at birth. The 

 same meatus may be directed much upwards and more or 

 less backwards, as in the Hare. No such part exists in the 

 skulls of Fishes. 



(3) The zygomatic part may in man's own class present 

 very considerable differences from the structure which exists 

 in him, and more considerable still in lower forms. 



Thus a zygomatic arch may be wanting, as in Centetes, the 

 Ant-eaters, and Manis. Both the anterior and posterior parts 

 of that arch may be well developed, and nevertheless may- 

 fail to effect a junction, as is the case in the Sloths and their 

 extinct allies ; or a junction may be formed by the help of a 

 process of the frontal, as in the Horse. 



In forms below Mammalia (as e.g. in Birds, the Crocodile, 

 c ) we often find a zygoma formed by the help of a bone 

 termed the qiiadrctto-jugal, which connects the malar in front 

 of it with that bone which supports the lower jaw, and which 

 is termed the quadrate. Besides this zygomatic arch a second 

 and superior zygoma may exist (as in most Lizards and in 

 some Birds, e.g. the Macaw " Calyptorhynchus "), formed by 

 the union of the squamous or zygomatic element of the tern- 



