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Show 276 ON THE STRUCTURE OF THE SKULL. that of the jaws, is soon arrested, and in addition the position and relations of some of the cranial bones become altered. In the smallest fcetuses of the lesser Fin-back (Pterobalmna minor), for example, the parietal region is occ~pie~ ~y the inte.rparietal bone and the great fontanelle whwh hes Ill front of It. In larger fcetuses the fontanelle becomes closed b~ the progressive backward growth of the frontals, but the extension of the bones does not cease with their contact. The parietals grow over the inter-parietal and spread over it until they meet in the middle line. Hence the inter-parietal is eventually visible only in the interior of the skull. Anteriorly, the parietals grow over the frontals almost to the same level as the nasals, and thus conceal the share which the frontals take in the formation of the roof of the skull. But, at the same time, the supra-occipital extends from behind over the parietals; so that, at length, in that region which, in the youngest fcetus, was covered only by the interparietal, three bones-the inter-parietal, parietal, and supraoccipital- are superimposed. The skulls of the other great division of the Cetacea, the Delphinoidea-or ,Dolphins, Porpoises, and Cachalots-are almost all distinguished by their very marked asymmetry. In the Cachalot, or spermaceti Whale (Physeter), for example, the right premaxilla is much longer than the left, extending far back upon the right frontal, while the left does not reach the left frontal; the left nostril, on the other hand, is much more spacious than the right (Fig. 110, A). On the base of the skull (Fig. 110, B) the pterygoid bones unite in the middle line and prolong the palate, as in Myrmecophaga and Ornithorhynchus. When they and the palatine bones are removed, the axis of t~e lower part of the ethmoid is seen to continue that of the basicranial bones, which are, as usual, quite symmetrical. Superiorly, however, the ethmoidal plate is twisted over to the left side, and deeply grooved on the right side to form the inner · wall of the small right nostril. 'rhe vomer, which embraces the ethmoid and the presphenoid below, ,is also asymmetrical posteriorly, presenting a long and shallow lateral excavation, on the left side, and a short and deep one on the right. The maxillre are correspondingly unsym- 'l'HE SKULLS OF MAMMALIA. 277 metrical in the region of the nasal aperture b t 1 h , u e sew ere t 11 ey are .p r. etty nearly symm.e trica. l. But it is the nasa 1 b ones wh I'C h exhibit the greatest distortiOn the left and · ht b · unequal I.n si.z e and di.s similar i'n form. r1g mng very The jugal and the lachrymal commonly become h 1 d Th b · · · t 1 . anc y ose . e asl-ocCipi a '. as In. the Balmnoidea, gives off a lateral downward process, whiCh u1utes behind (FI'g· Ill) 'th ' · , WI an out- Fig. 111. Fig. 111.-~Jongitudinal and vertical section of the skull of the White Whale or Beluga (~elpln_napterus). * marks a distinct bony element interposed between the alisphenoid t e par1etal, and basi-cranial axis; a, the ossified falx. ' ward p:olongation of the ex-occipital, and, in front, with the ptery~md, to. co.nstitute the inner wall of a deep chamber for the tympana-periotic. But the roof of this chamber is chiefly f?rmed by the very large alisphenoid, which extends outwards to unite with the frontals, parietals, and squamosals, and backward~ to the ex-occipitals. The aperture which is left betvveen ~he hind.er edge .of t~e alisphenoid, the ex-occipital, basi-occipital, an~ ~asi~sphenoid, Is exceedingly small, so that the tympanaperiOtic Is still more shut out from the cranial cavity than in Baltena. In Hyperoodon and Orca the aperture is still further reduced; but this peculiarity cannot be said to be a distinctive ?haracter of the Del phinoid skull, as in P latanista the aperture Is l~rg~, and the periotic appears in the interior of the cranial cavity In the ordinary way. |
