Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/469

Rh SKULL.] REPTILES 451 There is generally a superior zygomatic arcade formed by the junction of the postfrontal with the squamosal ; and, very generally, the orbit is enclosed posteriorly by a junction of this postfrontal with the malar. The basi- sphenoid sends down (as in Hatteria) two processes to abut against the pterygoids, which again join the quadrate bones. An os transversum unites the pterygoid, palatine, and maxilla of either side. The maxilla is thus a fixed bone ; the premaxilla is generally single, and sends a median process backwards. The nasals, frontals, and parietals may be single or double. There is generally a parietal fontanelle. By very rare exception, as in Monopeltis, there may be two occipital condyles, that (median) portion which is formed from the basioccipital aborting. FIG. 11. Skull of Monopeltis sphenorhynchus (from nature). 1, dorsal aspect ; 2, ventral aspect ; 3, lateralj aspect ; 4, posterior aspect, ar, articular ; 6s, basisplienoid ; d, dentary;/, frontal; m, maxilla; n, nasal; oc, uc, occipital condyles ; of, occipital foramen ; pal, palatine ; pa, parietal ; pm, premaxilla ; ptg, pterygoid; q, quadrate ; so, supraoccipital ; sq, squamosal; v, vomer. The hyoid consists of two pairs of cornua, whereof the anterior is generally the longer, attached to a median portion from which azygous a bifold process may proceed both anteriorly and poste- riorly. In the Amphisbaenians the skull is more solidly and con- tinuously ossified than is usual in Lizards, though there is no columella and though the low ali- and orbito-sphenoidal regions are membranous. The parietal is not movably arti- culated to the exoccipital, and the facial part is solidly ossi- fied and not movable upon the hinder portion of the skull. The orbits are not enclosed behind, and there is no superior FIG. 12. Dorsal aspect of skuiiof ffeio- nrparlp derma horridum. f, f rental ; j, jugal ; arcaae. - /.lachrymal; m, maxilla; n, nasal; The Skull of Heloderma is P a > Parietal; pm, premaxilla; pr, . . ., . . pref rental ; ps, postfrontal ; pt, ptery- Very remarkable in that it has gold ; q, quadrate ; s, squamosal ; so, no zygomatic arch whatever, "upraoccipitai. and in that the pre- and postfrontal bones unite and exclude the frontal bone from the margin of the orbit. The skull of the Chamseleons has even more the aspect of an osseous scaffolding than has that of ordinary Lizards in spite of the absence of a columella. This is owing to the presence of a long supraoccipital arcade formed by ps a long posteriorly and inwardly extending process of the squamosal, which joins the adjacent side of the singularly prolonged and upwardly and backwardly extending pro- cess of the parietal, which itself is solid- ly united to an up- wardly extending process of the supra- occipital. Thus the facial part of the skull is not movable upon its occipital portion. The orbits are enclosed by bone, and there is a superior zygomatic arcade, the post- FIG. 13. Skull of Chamxleonvulgaris (from nature), frontal im'nintr thp a 9> an g ular ; "''. articular; bs, basisphenoid ; d, Ontai joining I dentary; j, jngal; m, maxilla; me, median eth- SquaniOSal behind moid; pi and p2, parietals; pt, palatine; pr, ,i i prefrontal; ps, postfrontal; pt, pterygoid; q, and. tiie Hialar quadrate; sy, surangular; so, supraoccipital; sq, front. The frontal bone is small and single. There are a pair of narrow nasals, but these do not form the boundary of any part of the an- terior nares, but are excluded therefrom by the junction of the prefrontals with the maxilla, and these two bones may be prolonged so as to form great horn-like processes. The pterygoids do not articulate with the quadrates, and there is no interorbital septum. The hyoid has its posterior cornua much the longer, and a bony median basihyal. . The Ophidians, like the Lizards, have skulls which are iu Opli formed on more than one type : (1) those of the dians '> ordinary wide-mouthed Serpents; and (2) those of the Serpents with a very narrow gape, such as we find, e.g., in Typhlops and Uropeltis. The wide-mouthed Serpents, or Eurystomata, have a skull which in some respects is much less completely ossified and more movable than in ordinary Lizards, while in other respects it is more ossified and less movable. Thus the lateral walls of the anterior parts of the cranial FIG. 14. Skull of Python seox (from nature), ar, articular; ca, columella auris, d, dentary ; /, frontal; m, maxilla ; p, parietal ; pm, premaxilla ; po, prootic ; pr, prefrontal; ps, postfrontal ; pt, pterygoid; q, quadrate; s, squamosal; t, transversum ; tb, turbinal. cavity are well ossified, so that the anterior part -of the skull is no longer movable on the occipital segment, while on the contrary the total absence of both zygomatic arcades, the non-union directly of the palatine with the vomer, the laxity of union of the premaxilla and maxilla, and especially the movable condition not only of the quadrate but also of the squamosal (often an elongated bone) from which it is suspended, all give an excessive mobility to the facial part of the skull. To this it may be added that, at the symphysis, the rami of the mandible are only united by soft, very extensible tissue. There is usually but one premaxilla, and that is edentulous. The palatines are usually connected with the maxilla by transverse bones, while the pterygoids connect them with the quadrate bones. There is no interorbital septum, and