254 ON THE STRUCTURE OF THE SKULL. 



it. The tympanic, however, forms only the outer part of the 

 posterior wall of the tympanum, the inner and posterior walls 

 of that cavity being furnished by a downward process of the basi- 

 occipital, while its inner and anterior wall is formed partly by 

 the pterygoid and partly by the alisphenoid. These two bones 

 enclose a great air-cell, which communicates freely with the 

 tympanic cavity behind. In front, it is closed by a thin bony 

 partition, which separates it from a second large air-chamber, 

 enclosed, partly by the alisphenoid and pterygoid, and partly by 

 the palatine. 



In the genus Mams there is a large bulla, formed altogether 

 by the tympanic, which, in moderately young skulls, at any 

 rate, is not anchylosed with the adjacent bones. 



The squamosal is an immense bone, extending from the ex- 

 occipital to the orbito-sphenoid, and entering into the lateral 

 walls of the skull for that extent. Its posterior part, dilated 

 and convex outwards, contains a large air-cell, which opens into 

 the roof of the tympanum by a wide aperture. The plane of the 

 periotic is nearly horizontal. It is a relatively small bone, and 

 only a small part of it appears on the base of the skull, behind 

 the tympanic bulla, the squamosal completely hiding it externally. 



Of the Armadillos, some, like Eujyhr actus, have a tympanic 

 bulla of the ordinary construction, with, occasionally, a very 

 long external auditory meatus ; while others, such as the nine- 

 banded Armadillos (Praojms of Burmeister), have a mere hoop 

 of bone open above, almost as rudimentary as that of Echidna. 



Or, if we turn to the perissodactyle TJngulata : — 



In the Rhinoceros, the periotic and tympanic early anchylose 

 together, but remain distinct from the surrounding bones, the 

 compound tyinpano-periotic being only wedged in between the 

 squamosal, ex-occipital, and other adjacent cranial bones, in 

 such a manner that it cannot fall out. The "pars mastoidea''' 

 is completely hidden, externally, by the union of the squamosal 

 and the paramastoid process of the ex-occipital over it. The 

 region itself, however, is very well developed, and is continuous, 

 internally and below, with a very strong, conical, somewhat 

 curved, styloid process, to the flattened, free base of which the 

 hyoidean apparatus is attached. 



