. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 463 separation. The two halves of the jaw are movably connected together in the eurjstomatoas Ophidii. Parker, W. K., Stracture and developnieat of the skull ia the Ostrich, tribe. Philos. Traus. 1866.—The same, Oa the stracture and development of the skull of the common Fowl. Phil. Trans. § 350. lu the Mammaliau skull the cartilaginoas prhnordial cranium is ordinarily developed in the basal region only, and limited to the early stages of development. That part of the skull which is derived from the cartilaginous cranium is in this
. Elements of Comparative Anatomy. SKULL OF VERTEBEATA. 463 separation. The two halves of the jaw are movably connected together in the eurjstomatoas Ophidii. Parker, W. K., Stracture and developnieat of the skull ia the Ostrich, tribe. Philos. Traus. 1866.—The same, Oa the stracture and development of the skull of the common Fowl. Phil. Trans. § 350. lu the Mammaliau skull the cartilaginoas prhnordial cranium is ordinarily developed in the basal region only, and limited to the early stages of development. That part of the skull which is derived from the cartilaginous cranium is in this group also to be distinguished from the parts which are developed from other elements, but it becomes intimately con- nected with them. As a capsule for the brain it is itself larger, and is enclosed by a larger number of bones. It is more distinctly divided into different segments than it is in the lower divisions, but this must be regarded as a secon- dary adaptation (§ 340). In the occipital seg- ment the lateral pieces (Fig. 252, 01) always unite with a part of the basi-occipital (Fig. 253, Oh) to form tbe posterior occipital condyles, by which they bound the foramen magnum, while superiorly they enclose between tbem the supra-occipital (Os). This latter may be shut out from the edge of the foramen magnum. The four pieces almost always fuse together, but they may remain lono- sepai-ate (Marsupialia). In many Mammals (several Marsupials, Ungulates, etc.) the exoccipitals send down long processes {pn) (Paramastoid processes). In the region of the auditory capsule separate ossifications in the cartilaginous portion can be seen in the earliest stages only. They form centres of ossification which partly correspond to the bones in Fishes and Eeptiles; these soon fuse into a single piece, the petrosal (Pe), the greater part of which gets to be placed at the base of the cranium as the skull grows out laterally. The lateral portion of the petrosal is ovei4aid by other bones, wh
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