252 ON THE STRUCTURE OF THE SKULL. 



versely concave ridge, constituting the posterior boundary of 

 the olfactory fossa. Laterally, the basi-cranial processes of the 

 frontals arch downwards for a short distance and unite with the 

 lateral masses of the ethmoid. 



In the Gorilla, the frontal bridge is much smaller than in 

 the lower Catarhines. The Chimpanzee approaches Man still 

 more nearly ; a triangular process of the presphenoid inter- 

 posing itself between the frontals and joining the ethmoid. 

 Sometimes, however, very small processes of the frontals just 

 unite over this junction. In the Orang, the frontals are widely 

 separated, as in Man. 



The epiotic, pro-otic, and opisthotic bones are always an- 

 chylosed into a single periotic bone in the Mammalia ; but they 

 unite with the other elements of the temporal bone, and with the 

 adjacent cranial bones, in very various modes, and the tympanic 

 cavity presents very different boundary walls in different Mammals. 



In the Beaver, as we have seen, the tympanic and periotic 

 bones are anchylosed into a single " tym'pano-'periotic" which 

 remains unanchylosed with the squamosal, and is easily de- 

 tached. In the Sirenia and in Cetacea (sooner or later) the 

 same anchylosis takes place, but the tympano-periotic is still 

 less firmly fixed in its place, and, in some Cetacea, does not 

 appear at all in the interior of the skull, in consequence of the 

 growth of the adjacent bones towards one another over it. 



The tympano-periotic of the Rhinoceros, Horse, and Sheep, 

 long remains unanchylosed to the surrounding bones, but is so 

 wedged in between them as to be practically fixed within the 

 walls of the skull. 



In Echidna and in Orycterojms the periotic, the squamosal, 

 and the tympanic remain perfectly distinct for a long time, if 

 not throughout life. 



The squamosal and tympanic of the Pig anchylose into a 

 single " ' squamoso-tympanic" which is firmly fixed to the ad- 

 jacent bones ; but the periotic remains free, and consequently 

 readily falls not out of, but into the skull. 



In the nine-banded Armadillo (Praopus) it is the periotic 

 and squamosal which are anchylosed, the tympanic remaining 



