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. 1869. 



§ 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, which are developed from 

 tlie metamorphosis of the branchial skeleton, and becomes converted 

 into the middle wall of the tympanic cavity, in which there is a 



'pm 



Fig. 252. Lateral view of the cerebral portion of the 

 Skullof a Goat. 0? Exocoipital. Os Supra- occipital. 

 Jp Interparietal. Pa Parietal. Pe Petrosal. 

 8q Squamosal. Ty Tympanic. Sph Basi-sphenoid. 

 As Alisphenoid. Ors Orbito-sphenoid. Fr Frontal. 

 Na Nasal. L Lachrymal. Jit Jugal. Mx Maxillare 

 superius. Pal Palatine. Pt Pterygoid, pm Para- 

 mastoid process, si Styloid process. 



