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Show 218 0~ THE STRUCTURE OF THE SKULL. surf :a ce t o tl 1e ax1' a] , or "Meckel's cartilage," of th. e mandib. le. In the young Tadpole, a line drawn from the mandibular .artiCu-lation to the auditory capsule makes an acute angle With tho basis cranii; but, as age advances, the angle becomes more and more open, until, in the adu~t Prog, !t is obtuse (Fig. 8.6), the articular surface for the mandible having passed far behind tho auditory capsule. Of course the width . of the gape increases par~: passu with this rota.tion of the mandi.b~la1~ suspensor. . A survey of the senes of the Amph~b~a from the perenni-branchiates upwards, shows, in a persistent form, those inclinations of the suspensor which are transitory in the F rog. Thus in the perennibranchiate Siren, Siredon, Proteus, and Menobranchus, the angle is acute; in the Sala1nander and Salamandroid Menopoma, it is nearly a right angle ; while, in the Frogs and Toads, and the ancient Labyrinthodonts, the angle is obtuse. In the lower Amphibia there is no girdle bone, the orbito-sphenoid and the prefrontals being usually represented by distinct bones. The frontals are distinct from the parietals, and the maxillary and pterygo-palatine arcades become im-perfect. Some of the Frogs and the Ooocilim-the snake-like apodal Amphibia-have the cranial bones expanded and anchylosed into a sort of shield, presen6ng apertur s only for the orbits and the nostrils; a process which is carried still further, by the addition of bones not known to existing Amphibia, in the extinct salamandroid members of the class, called Labyrinthodonts. 219 LECTURE XII. ON ~rHE STRUCTURE OF THE SKULL .. THE SKULLS OF REPTILIA AND AVES. THE skulls of those abranchiate Vertebrata which do not suckle their young, and are oviparous, or ovo-viviparous, present certain peculiarities of construction in which they all agree with one another, and differ from the branchiate Vertebrata on the one hand, and from the Mammalia on the other. Thus, the basi-occipital and the basi-sphenoid are always well developed, and the former furnishes a large part of the occipital condyle, which is single and central. There is no parasphenoid, or median membrane bone, under-lying the base of the skull. · The lower jaw, each ramus of which is composed of several pieces, articulates with the quadrate bone, as in the branchiate Vertebrata; but the quadrate bone articulates directly with the cranial wall, and is not separated from it by any structure representing the hyomandibular bone. It ~ay probably be added that the basi-sphenoid is formed by the unw~ of three ossifications of cartilage-one supero-median and two Infero-lateral (the basi-temporals of Mr. Parker); but further research is required before this generalization can be regarded as firmly established.* . * The cau f wn expressed m· the text seems to be no longer necessary, as my fnend Mr. Parker, who possesses a remarkably extensive knowledge of the details of the structure and development of the vertebrate cranium, informs me that he has now. f?und "the median basi-sphenoid and the symmetrical basi-temporals in Ophldians, Anguians, Scincoids, Iguaniaus, Geckos, Chamreleons, Cyclodonts, Lacer- |