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Show 274 ON THE STBUOTURE OF '!'HE SKULL. become greatly elongated and very rugged, the tympanic also acquiring a very large size (Fig. 109). The vomer is a very long and large bone, deeply grooved above for the ethmoidal cartilage, which extends down wards and forwards between the premaxillre. and the maxilloo to near the anterior end of the snout. Its expanded upper and posterior end unites with the basi-sphenoid in the rniddle line, and with the pterygoid laterally. In front of the basi-sphenoid it embraces, not a distinct presphenoid (as in Pterobahena, according to Eschricht ), but the inferior surfaces of the or hi tosphenoids, which are very thick; and, being applied together by their flat median faces, apparently replace the proper pre-sphenoid. Both these bones and the alisphenoids are small, and almost confined to the base of the skull. The supra-occipital and inter-parietal are united together, and completely overlap and hide the parietals in the roof of the skull. The separate frontals only enter into the anterior wall of the skull, and between them and the orbito-sphenoids an oval aperture is left, doubtless diminished in the recent state by tlte ethmoidal cartilage. Laterally, the frontals are prolonged outwards and backwards into two great supra-orbital processes, which nearly meet the zygomatic processes of the squamosal. The short jugal bones, absent in the specimen figured, extend in the Balt:enoidea from the zygomatic process to the anterior and external angles of the supra-orbital 11rolongations, and are distinct from the lachrymals. The pterygoids are completely separated by the palatines (Fig. 107). In front of the latter the maxilloo almost wholly exclude the premaxillaries from the palate, while they send great processes obliquely outwards and backwards, in front of the supra-orbital prolongations of the frontal. The long premaxillre, on the other hand, pass upwards and backwards on eaeh side of the elongated and syn1metrical nasals to meet the frontals, and exclude the maxillrn altogether from the anterior nares. The rami of the lower jaw are very narrow, and so much arched outwards as to be able to enclose the baleen plates attached to the upper jaw when the mouth is shut. THE SKULLS OF MAMMALIA • 0 275 Eschricht has described witl I the skulls of the Bahenoid,ea 1lmuc 1. care, the changes which unc ergo In pass· . f to the adult condition J·ustly , 1 . mg rom the footal ' remar ung th t th I I a large footus is more differe t f h a e s tu l of even . . n rom t at of the d It h skulls of distinct species of th . a u , t an the one another. e same genus of Whales are from The growth of the walls of th . . that of the external prolongation: c;a~Ial cav~ty relatively to 0 t 1e cranial bones and to A Fig. 110. B c Jii- Sa F' I Ig. 110.-A~ upper, B, under and 0 'd . f I . Museum of the Ro ·al Coll~ e ' SI e ~lew~, o t le skull of a fretal Cachalot in the Fig. A.-N' left jy,, .· I g- of .sur7;ons.. I he nasal bones are not represented in down from its 't . lug lt, n~stnl.. I he hmder extremity of the jugal, Jtt, has fallen na ma connectiOn With the zygomatic process of the squamosal. T 2 |
