OCR Text |
Show 270 ON THE ST RUCTURE OF THE SKUJ.JJ.J. . d d alisphenoid vary in size. frontals. The orbito-sphendm. fia~1 ly fixed to the side of the 1 · 1 ge an IS rm The squamosa IS . ar ' h . ll of the cranial cavity. The skull, forming part of t ed ":'at one bone with the bullate . · lly anchylose In ° . penotw,· usn a times enters 1a rge1 Y into the wall of the cranium, tymp~niC, s~me lmost altogether excluded therefroln by the sometimes ~s a 'd nd other adjacent bones, which send parietal, abspheno~ ' a rolongations over It. d t £ p The maxi'1 1 ary apparatus is greatly elongate ' so as o orin Fig. 107. Fig 107 .-Upper, under, and side vJ. ews of the skul l o f ~ [Ir e t ~ 1 Wthha lebaolnaet inWeh baolen e( Bisa lnccecni'-l dustmlis). The jugal bones are absent, In the un er new e P' dentally markerl Pt instead of Pl. 'l'HE SKULLS OF MAMMALIA. 271 a kind of beak. The premaxilloo enter into the upper and inner part of the whole length of this maxillary beak, but contribute little or nothing to its palatine surface and lateral boundaries, which are formed mainly by the 1uaxilloo. 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 produeed outwards, and the processes 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-o.ccipital are always prolonged do·wnwards 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 cham her, the anterior and outer boundaries of which are furnished 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 Balmnoidea, or" Whalebone Whales," the symmetry of the skull is undisturbed, though there may b~ a slight inequality of the maxilloo. The skull of the footal Balmna at£stralis, represented in Fig. 107, is perfectly syinmetrical. Each lateral edge of the broad and flat basi-occipital is prolonged dmvnwards and outwards into a broad process, concave outwards and convex inwards, the inferior edge of which is free, w bile the hinder edgA unites with the ex-occipital, and the front edge with the pterygoid, to form the inner wall of the funnel-shaped chamber which lodges the tympano-periotic bone. In front, this chamber is Lonnded 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 anrl roofed |