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Show 272 ON 'rHE STRUCTURE OF THE SKUJ.JL in by the squamosal. Between these .bones there is left, at ~he apex of the chamber, a considerable 1rregular aperture, which communicates with the eranial eavity. The anterior and outer part of the under-surface of the squamosal is produced downwards into a great trihedral pillar, the obliquely truncated inferior face of which bears the articular surface for the mandible ( Gl, Fig. 107). Behind this the squamosal presents a comparatively low wedge-shaped ridge (a, Fig. 108), between which and the "trihedral pillar'' is a groove; while behind it, or between it and the ex-occipital, there is a deeper and wider transverse channel. Fig. lOS. The periotic bone is irregularly triangular; the apex of the triangle, turned in wards and forwards, is thick and rounded, the anterior, posterior, and outer edges being thinner and more or less irregular. ~rhe upper sn1ooth and concavo-convex surface of the Fig. 1 08.-Eularged view of the chamber periotic adjusts itself to the under-which lodges the left tympano-periotic surface of the squamosal, where it bone of the fretal BalcEna aust?·alis.- a, the "wedge-shaped ridge,, of the forms the roof of the funnel-shaped squamosal; C, the aperture which leads into the interior of the skull; cavity. The apex of the periotic, f.o, fenestra rotunda. however, projects beyond this, and incompletely divides the irregular aperture aboYe mentioned (b, Fig. 108) into an anterior division, whieh corresponds with the foramen ovale and fora~rnen lacerum medium, and a posterior which answers to a foramen lacerum posterius. The under-surface of the periotic, much more irregular, is divisible into three regions: an outer anterior; an outer posterior; an internaL The first and second are separated by a deep triangular notch in the outer margin of the bone, into which the inner end of the wedge-shaped ridge of the squamosal is received. The first, broad and short (P·rO), presents a rough surface in front, with which the tympanic articulates, and eventually anchyloses ; and behind, a concave surface, which, entering into the roof of the tympanic cavity, answers to the THE SKULLS OF MAMMALIA. 273 tegmen tymty')ani TI . :r . le second' narrower elono- d . fits Into the transverse cl a 1 b I . ' bate ' and pnsmatic lc nne e nnd the wed h ' cess (a). It con·esponds with tl ge-s aped pro-le pars mastoide d · ont~r extre1nity appears 011 tl i . a, an Its rough tl le ex el'lor of the k 11 b ~ 1e squa~osal and ex-occipital. s u , etween The Internal division, convex and ro . by the pro-otic and opisthot. • d unded below, Is formed · h 1 Ic, an presents a la ·o· • Wit t 1e .fenestra rot·unda (jo) .t . 1oe promontory fenestra ovalis and Fallopi~a· on 11 s post.e~·wr surface, while the n cana are VISibl · The ty1npanic bone (Fio· 109 T ) . 1 e upon Its exterior. form, verv thick intern~il ' dyb Ils arge, and scroll-like in " y an e ow and tl . b externally, where it presents th ' nn a ove and e aperture of the t tory meatus. It is by this th. · · ex erna1 audi- . In upper and outer d 1 It eve·an tually anchyloses with tl t . e ge on y, that le egmen tymty')an. d mastoz ea, and hence as its s b t . :r ~ an pars read1. ly breaks off. ' n s ance Is very d ense an d b n.t tle, In the adult Whale the tegmen tymty')ani a d . :r n pars mastoidea Fig. 109. . , Ty FJg. '09 .-" Ear bones " 0 f h t e adlllt Balcena australzs Tl . . . . from within. the lo\v f . . !Je nppel llgnre gn'c:s the view ' · er, rom Wit 1out. T |
