THE SKULLS OF MAMMALIA. 271 



a kind of beak. The premaxillae enter into the upper and 

 inner part of the whole length of this maxillary beak, bnt 

 contribute little or nothing to its palatine surface and lateral 

 boundaries, which are formed mainly by the maxillae. The 

 latter bones are always prolonged over, or in front of, the 

 supra-orbital processes of the frontals. 



The imperforate lachrymal is small, and sometimes coalesces 

 with the jugal. 



The nasal bones are always short, sometimes rudimentary ; 

 and the palatine bones are so disposed that the posterior nares 

 are situated almost vertically under the anterior nares. 



The squamosal bones are produced outwards, and the pro- 

 cesses thus formed approach, or come into contact with, the 

 posterior part of the supra-orbital processes of the frontals, 

 which they separate from the jugal. Inferiorly, these processes 

 support the glenoidal facets for the condyle of the lower jaw. 



The sides of the broad basi-occipital are always prolonged 

 downwards into free plates, which are concave outwards. These 

 plates join the pterygoids in front, and the ex-occipitals behind, 

 and so constitute the inner and posterior walls of an auditory 

 chamber, the anterior and outer boundaries of which are fur- 

 nished by the alisphenoid and the squamosal. In this chamber 

 the tympano-periotic is lodged, sometimes quite loosely, at 

 others fixed firmly in by interlocking sutures. 



In the Balsenoidea, or " Whalebone Whales," the symmetry of 

 the skull is undisturbed, though there may be a slight inequality 

 of the maxillae. The skull of the foetal Balmna atistralis, repre- 

 sented in Fig. 107, is perfectly symmetrical. Each lateral edge 

 of the broad and flat basi-occipital is prolonged downwards and 

 outwards into a broad process, concave outwards and convex 

 inwards, the inferior edge of which is free, while the hinder edge 

 unites with the ex-occipital, and the front edge with the ptery- 

 goid, to form the inner wall of the funnel-shaped chamber which 

 lodges the tympano-periotic bone. 



In front, this chamber is bounded by the pterygoid and the 

 squamosal, and between and above them, for a small space, by 

 the alisphenoid ; behind, it is constituted almost entirely by the 

 ex-occipital, while, externally and above, it is bounded and roofed 



