THE SKULLS OF MAMMALIA. 253 



rudimentary and free ; and the Opossums and the Tapir exhibit 

 a similar arrangement. 



Other Mammalia, such as the Camivora and Primates, have 

 the squamosal, tympanic, and periotic all anchylosed together 

 into one " temporal bone." 



Even in one and the same order the constitution of the 

 tympanic cavity exhibits the most remarkable differences. To 

 take the Edentata as an example : — 



In the Oryderopus the walls of the tympanic cavity have a 

 wonderfully reptilian arrangement ; the periotic is very large in 

 proportion to the other bones of the skull, and its plane presents 

 comparatively little inclination, so that its exterior face looks 

 more outwards than downwards. A large part of its posterior and 

 outer face is seen, as a pars mastoidea, upon the exterior of the 

 skull, between the supra-occipital, the ex-occipital, and the squa- 

 mosal, but there is no distinct " mastoid process ;" below, the peri- 

 otic comes into contact with the basi-occipital and basi-sphenoid ; 

 in front, with the alisphenoid. The latter bone is strongly convex 

 outwards, so as to present a posterior, as well as an external, face ; 

 the posterior face forms the front wall of the tympanum, and ex- 

 hibits a somewhat deep excavation, or alisphenoidal air-cell. 



The squamosal, a very large bone, is divided by a well- 

 marked ridge into an upper face, which constitutes part of the 

 roof, and an outer face, which forms a portion of the lateral 

 wall, of the skull. The latter enters into the outer and upper 

 wall of the tympanum ; the former, very thin, constitutes the 

 roof of that cavity, abutting internally upon the supra-occipital 

 and parietal. The Fallopian canal is open for the greater part 

 of its extent, and a hook-like osseous process, which overhangs 

 its outer and posterior part, gives attachment to the hyoid. 



The tympanic is a strong hoop of bone, incomplete above,, 

 and much shorter anteriorly than posteriorly. By its expanded 

 anterior end it articulates by an interlocking suture with the 

 squamosal. The thin posterior end is free. 



In Myrmecophaga tetradactyla (and essentially the same 

 arrangement obtains in the great Ant-eater), the squamosal, as 

 in Oryderopus, enters largely into the wall of the cranial cavity ; 

 but the tympanic, which is large and bullate, is anchylosed with 



