THE SKULL.--The skull in the Mammalia differs from that of the lower Vertebrata in a number of important features, which will be enumerated in the following brief sketch of its structure. In the first place, the skull is a more consolidated whole than in reptiles; the number of elements entering into its formation is less, and they are on the whole more firmly welded together than in Vertebrates standing below the Mammalia in the series. Thus in the cranial region the post- and pre-frontals, the post-orbitals and the supra-orbitals have disappeared, though now and again we are reminded of their occurrence in the ancestors of the Mammalia by a separate ossification corresponding to some of the bones. Nowhere is this consolidation seen with greater clearness than in the lower jaw. That bone, or rather each half of it, is in mammals formed of one bone, the dentary (to which occasionally, as it appears, a separate mento-Meckelian {26} ossification may be added). The angular, splenial, and all the other elements of the reptilian jaw have vanished, though the numerous points from which the mammalian dentary ossifies is a reminiscence of a former state of affairs; and here again an occasional continuance of the separation is preserved, as the case observed by Professor Albrecht of a separate supra-angular bone in a Rorqual attests. Among other reptilian bones that are not to be found in the mammalian skull are the basipterygoids, quadrato-jugal, and supratemporal. A few of these bones, however, though no longer traceable in the adult skull save in cases of what we term abnormalities, do find their representatives in the foetal skull. Professor Parker, for example, has described a supra-orbital in the embryo Hedgehog; a supratemporal also appears to be occasionally independent. [Illustration] FIG. 15.--Head of a Human embryo of the fourth month. Dissected to show the auditory ossicles, tympanic ring, and Meckel's cartilage, with the hyoid and thyroid apparatus. All these parts are delineated on a larger scale than the rest of the skull. _an_, Tympanic ring; _b.hy_, basihyal element; _hy_, so-called hyoid bone; _in_, incus; _md_, bony mandible; _ml_, malleus; _st_, stapes; _tp_, tympanum; _tr_, trachea; I. (_mk_), first skeletal (mandibular) arch (Meckel's cartilage); II. second skeletal (hyoid) arch; III. third (first branchial) arch; IV. V. fourth and fifth arches (thyroid cartilage). (From Wiedersheim's _Structure of Man_.)