104 ELEMENTARY ANATOMY. [LESS. 



It may develop, as in Fishes, a post-orbital process of its 

 own. 



Its junction with the malar is not found anywhere out 01 

 man's class. 



The squamosal of some Birds (as the Ostrich) may send 

 down a process, the representative of which is more con- 

 spicuous in the Frog, and becomes separated off from the 

 superior portion in the Eft. It is this lower portion which ib 

 the pre-operculum of Fishes. 



FIG. ioi. SIDE VIEW OF THE SKULL OF A RATTLESNAKE (Crotalns). 



a, articular bone of lower jaw; bo, basi-occipital ; </, dentary;_/", frontal ;_/", 

 pre-frontal ; me, median ethmoid ; inx, maxilla ; o, pro-otic ; /, parietal ; pa, 

 para-sphenoid ; pt, pterygoid ; px, pre-maxilla ; git, quadrate ; qs, squamosal. 



The petrous and mastoid elements (petro-mastoid) of the 

 temporal bone require a more lengthened notice. 



In the first place, it may be observed that the distinct- 

 ness of these united elements is shown in many Mammals 

 where the petro-mastoid bone remains unanchylosed and 

 separate from both the squamosal and tympanic elements 

 of the temporal bone. This is the case, e.g., in the Pig. It 

 may be united with the tympanic element, but not with the 

 squamosal, as in the Hare and the fully adult Porpoise, 

 being in the latter only united to the rest of the skull by 

 ligamentous attachment. 



Still, in all man's class, what we have seen to be the con- 

 stituent parts of the petrous and mastoid elements always 

 cohere into a single mass before they unite with other cranial 

 elements, while such a complete primitive union never takes 

 place in any Vertebrate not a Mammal. 



The very prominent mastoid process of man is peculiar to 

 him, as, though in many other Mammals it is more or less 

 developed, it is generally rivalled or surpassed in them by 

 the elongated process of the occipital bone before mentioned. 



Even in the highest Apes the degradation of the mastoid 



