OCR Text |
Show 20() ON THE S1'RU<.Yl'URE OF THE SKULL. The superior and middle turbinal bones are greatly flattened fro1n side to side, and unite below and internally with the lam,ina Fig. 105. Fig. 105.-Longitudinal and vertical section of the skull of a Seal (Pltoca vitutina). The premaxilla is absent. perpendicularis, or proper ethmoid, so that all direct coininlnlication with the superior and middle meatuses of the nose is shut off below. The inferior turbinal, on the other hand, is exceedingly large and complex in its structure. The orbito-sphenoids are large and, ascending upon the front wall of the skull, unite anteriorly behind and below the cribriform plate, so as to hide nearly the half of the ethmoid when the base of the skull is regarded from above. The presphenoid is relatively small. Less than half the length of the frontal bones enters into the upper wall of the cranial cavity, the rest being devoted to the roof of the nasal chambers. This part of the frontals is very much narrower than the other, and is bent down at the sides, so as to form two broad thin plates, which wall in the superior and middle spongy bones, articulate below with the vomer and with the palatine, and take the place of the os planum. The lower edge of the parietal unites with the front part of the alisphenoid and with the ex-occipital, leaving a great inferolateral space, which is filled up in front and above by the squamosal, and behind and below by the periotic. The squamosal is relatively a small bone, but the periotic and the tympanic, which are anchylosed with it, are very large. A swollen pars mastoidea appears on the exterior of the skull, and is hollowed internally by a cavity which opens into the cranium, 207 THE SKULLS OF MAMMALIA. and extends under the a t . C•i rcular canals. , c: n erwr and po st en.· o r verti.c al semi- . The tympanic forms a ver . thick bull . Into an auditory meatus It ~ :fi 1 a, piolongef.l externally . . . Is rm y auchyl d . h 1 otiC regions of the peri'ot1· c an d W.i t h the ose IW it t 1e pro-rest of its extent it is only a 1. d squamosa , but for the the periotic. It is pierced byppthie to, a?-d not anchylosed with, Th e carotid canal ·1 e anchylosed squamosal ' pen.· ot 'I c and tym· · easi y detached from the walls of the k . panic are very from the upper jaw. · s ull, as Is the premaxilla The skull of the Dugon . ( R z· . peculiarities of the cranial g .{.'a ~cor~, Fig. 1 06) presents the d . con1ormatwn of M 1 f or er s~renia in a very m·a r1 r e d 1.{.' 0rm The b a· mma. s o .t he almost flat above but th' k · asi-cranial axis is . . ' very Ic . Th t b bas1-ocmpital and the b asi.- sp 11 eno1' d perse · t su bu re he tween the the basi-sphenoid and th . . . IS s, ut t at between as is that between the ~ prlespl~ednmd Is completely obliterated, piesp 1enoi and the ethmoi'd , wl u.c l 1 I ast Fig, 106. Fig. 106.-Longitud"m a 1 an d verti.c al section of the skull f D . 0 a ugong (Hahcore Indicus ). has the form of a stout bon I . - . posterior edge, or criBta alli y p ate, with an .almost vertical frontals is very narrow f g b. :D The upper median part of the not more than th r~m. e ore backwards, so that they cover ethmoid and ap e pbos erwr 17alf of the upper edge of the ' pear ut very h ttl th cavity; laterally and belo e on e roof of the cranial produced forwards d w, they are much expanded, and of the skull I·s f ~nh doubtwards. T.he greater part of the roof tn·nis e y th e pan.· e ta1 s , the longest antero- |