GENERAL STRUCTURE. 1 25 I 



In adult Mammals, as in the Sauropsida, the whole of the primi- 

 tive cartilaginous cranium is replaced by extensive ossifications, 

 except in the ethmoidal region ; and these bones, with the excep- 

 tion of those of the mandible, hyoid arch, and internal auditory 

 apparatus, are similarly articulated together at their edges by suture. 

 As age goes on there is generally a tendency to the obliteration of 

 these sutures, this being most marked in Mam's. In all cases the 

 parasphenoid has ceased to exist as a distinct ossification. It has 

 already been observed that the hinder part of the cranium articulates 

 with the first, or atlas, vertebra by two exoccipital condyles, and 

 also that each of the two rami of the mandible is composed of a 

 single bone articulating at its proximal or hinder extremity with the 

 squamosal of the cranium. Owing to the complete incorporation of 

 the squamosal and parietal in the walls of the brain-case there is 

 (as in the Birds) no superior temporal arcade ; but an inferior, or 

 zygomatic, arcade is nearly always present ; and, as in Crocodiles 

 (although not in Birds), forms the lower border of the orbit. This 

 inferior temporal arcade, or arch, differs, however, from that of most 

 Sauropsida in that its jugal element articulates directly with the 

 squamosal, in which respect it accords with that of the Dicynodont 

 Reptiles, and should be known as a squamoso-maxillary arcade. 

 According to the view of Professor Huxley, the Sauropsidan quad- 

 rate has been taken up into the inner ear to form the malleus. 

 More recently, however, other writers have taken a different view, 

 and, according to Dr Baur, the representative of the quadrate is to 

 be found in the zygomatic process of the squamosal, with which the 

 jugal articulates ; the quadratojugal being also represented at the 

 junction of these two bones. Dr Gadow, however, disputes this 

 view, and finds the representative of the quadrate in the tympanic 

 ring, this determination according with the view here taken as to 

 the homology of the zygomatic arcade, which appears to have no 

 quadratojugal element. 



Again, when, as in the Primates and many Ungulates, the orbit 

 is bounded posteriorly by a bony postorbital bar, the ascending pro- 

 cess of the jugal articulates directly with the frontal, without the 

 intervention of the postorbital or postfrontal, which forms such a 

 conspicuous feature in the Reptilian skull. 



In all cases the premaxillae, maxillae, and palatines develop in- 

 ferior palatal plates which meet in the median line below the nasal 

 passage, and thus completely separate the latter from the cavity of 

 the mouth. Except, however, in the Anteaters (Myrmecofihaga), in 

 one genus of Armadillo (Tatusia), in certain Cetaceans, and in one 

 species of Ifycenodon, this flooring of the nasal passage does not 

 extend backwards to include the pterygoids, as it does in modern 

 Crocodilians. The palatines are always placed behind the maxillae. 



