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Show 264 ON THE BTHUO'l'UB.E OF 'l'HE SKULL. are adjusted by a broad posterior face to t~e frontals at N~*. The large premaxi11aries ascend along the sides of the anterwr nasal aperture to the nasal bones~ bu~ are aln:1ost excluded f~·om the palate, inferiorly, by the maxlllanes ; then alveolar portwn, however, is very large and long., and this circu1nstance, together with the shortness of the nasal bones, throws the anterior nares, in the dry skull, almost to the top of the head. As the palatine processes of the maxillaries and palatines are, at the same time, relatively short, the posterior nare~ are situated but little behind the anterior nares, and thns the axis of the nasal passage forms a large angle with the basi-cranial axis. The la('hrymal is a very small, though distinct, bone. In the footal Elephant here· described the space between the two tables of the skull is moderate, and is filled with a spongy diploe; but, with advancing age, the interspace between the tables in the frontal, parietal, and supra-occipital increases until it equals or exceeds the depth of the cranial cavity, and the diploe is replaced by vertical plates and pillars of bone, between which air-cavities extend back from the frontal sinuses and nasal passages. The skull of the Elephant resen1bles that of the .Pig in many of its most important and characteristic features, and, through the Pig, its affinities are traceable to the other Ungulata. Of these, the skull of the Tapir resembles it most in some respects, such as the shortness of the nasal bones and of the palate ; the consequent large angle which the axis of the nasal passages makes with the basi-cranial axis; and the prolongation downwards and forwards of the frontal bones. Fig. 104. Fig. lO.J..-Side view of t.he skull of a Calf.-pmcl, the paramatitoid process of the ex-occipitnl, 'l'HE SKULLS OF 1\lAMMALIA. 265 On the other hand, some ·Ruminants carry to an oxtremo the development of the frontal into a great supra-orbital arch, its extension backwards in the 1niddle line, and the concomitant expansion of the supra-occipital forwards ; so that the parietals of the Ox, for example, are reduced to a comparatively narrow band in the middle line, while they expand widely in the temporal fossoo (Fig. 104). The crania of the purely aquatic Mammals, such as the typical Seals, the Sirenia and the Cetacea, exhibit a certain similarity of character in the midst of very wide and important differences. The basi-cranial axis is either flat or slightly curved upwards at its anterior and posterior extremities. ~rhe olfactory and occipital planes are vertical, or nearly so. The squama occipitis, alone, or united with large inter-parietal elements, extends upon the vertex of the skull between the parietals, and approaches, or even reaches, the frontals, so that the parietals are very much shorter antero-posteriorly than at the sides and below. The frontals take but a small share in the formation of the roof of the cranial cavity; the nasals are relatively short, the anterior nasal aperture relatively large, and the posterior often situated far forwards. The prefontals, or lateral masses of the ethmoid, are s1nall or rudimentary. The tympanie and periotic are always anchylosed together, and, whether connected or not with the squamosal, are more easily detachable from the skull than usual. The Seals are extreme aquatic modifications of the carnivorous type of cranial structure; the Sirenia, of the ungulate type. ~rhe Cetacea present resemblances to both. In the common Seal (Phoca vitulina) (Fig. 105) the cranial cavity is exceedingly broad and spacious, 'and the cerebral extends far further back over the cerebellar chamber, and is much larger in proportion to it, than is usual in Carnivora. There is a strong bony tentorium, and an osseous falx is more or less developed. The basi-cranial axis, very thin and broad, is curved, so as to be concave from before backwards. The synchondrosis between the presphenoid and basi-sphenoid persists. |
