Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/797

Rh AMPHIBIA 753 ankylosed. The anterior face of the style usually presents two concavities (one in Bombinator and some other genera), which articulate with the corresponding convexities on the posterior face of the A B centrum of the sa cral vertebra. The number of the verte brae in the spinal column of the Uro- dda and Peromela varies very much. In the long-bodied Urodela and Pei O- rifl - 3 - A. caudal vertebra of Afenopoma. N, neural 7 ,1 i arch; C, centrum; Z, Z*, pne- and post-zygapo- mela tney may De physes; T.p. transverse process; S.V, sub-verte- vei V numerous tral arch. A, lateral; B, posterior view. According to Cuvier, Ca dlia has 230; /Siren, 99; Amphi- uma, 75 ; in Menobranchus there are 18 praa-sacral and 25 caudal; in Salamandra, 15 and 2G ; and a similar varia tion appears to have obtained in the Labyrinthodonts. On the other hand, in the Anura the number of vertebras (ex cluding the coccyx) is very constantly nine ; though this number undergoes an apparent reduction, in some cases, by the ankylosis of the first and second vertebrae (Cera- tophrys dorsata, Pipa, Dactylethra, Breviceps), and in others by that of the sacral vertebra with the coccyx (Pipa, Dactylethra, Breviceps, Pelobates). In the carboniferous Labyrinthodont, Archegosaurus, the notochord appears to have persisted throughout life, and the ossification of the centra of the vertebrae to have gone no further than the development of bony rings, such as those with which the ossification of the centra of the verte bras of a tadpole commences. The Cranium. The skull is always very depressed, and is usually broad in proportion to its length, though, in this respect, there is considerable variation, the skulls of Proteus, Menobranchus, and Ampkiu?)ial&amp;gt;eiug narrow, when compared with those of Siredon, Mcnopoma, and the Anura. The occipital foramen is situated in the middle of the posterior face of the cranium, and there are always two occipital condyles. The long axis of the suspensorium, or pedicle by which the mandible is connected with the side- walls of the brain-case, varies much in its direction passing obliquely downwards and forwards in the lower Amphibia and in the larval condition of all, but swinging back until it stands out at right angles to the axis of the skull, or becomes directed downwards and backwards, in the higher Amphibia. The suspensorium is almost immov able upon the skull, being clamped thereto by the squa- mosal bone, besides being, as a general rule, united with some part of the wall of the skull by synchondrosis. The &quot; primordial skull,&quot; or chondro-cranium, usually remains, to a great extent, unossified, even in the adult. In the Urodela, the hyoidean arch is always connected by strong ligaments with the suspensorium; but, in the Anura and in the Peromela, it becomes completely detached from the suspensorium, and may be free (Peromela}, or acquire a new attachment to the periotic region of the skull in front of the fenestra ovalis (Anura). The bones which are always present in the Amphibian skull are the exoccipital, pro-otic, parasphenoid, vomer, parietal, frontal, squamosal, premaxillary, palatine, quad rate, dentary, splenial, and angular. The basi-occipital and the basisphenoid are always absent, or are repre sented by mere partial calcifications of the chondro- cranium. There is always a fenestra ovalis closed by a stapes. The branchial arches do not exceed four pairs in number, and, in the perennibranchiate Amphibia, there are never fewer than three pairs. The skull of the Frog (Figs. -1-7), as the most accessible member of the group, and that, the development of which has been most carefully studied, may be taken as the starting-point from whence to follow the various modifica tions of the Amphibian skull. At the sides of the occipi tal foramen, it presents two large exoccipi- JHH.fl&V x tal ossifica tions (E. 0.), which bear the prominent oc cipital con dyles, and, in old specimens, may meet in the middle ventral line. Dorsally, how ever, they re main sepa rated by a narrow tract of cartilage, which may be come more or less calcified. External to the condyles, are the fora- mina,by which the vagus and glosso-pharyn- geal nerves emerge from the cranial cavity; and, beyond these, the bones ex pand outwards and forwards, so as to em brace the pos terior half of the fenestra ovalis, while above, they enclose the greater part of the posterior vertical semi circular canal. The cartilage which incloses the summit of the arch of that canal, however, ap pears always to remain un- ossified, and its place is OC- An. angulare; D. dentale. V. foramen of exit of the trige- . j -I minal; II. of the optic; X. of the pneumogastrie and CUpied Dy a glosso-pharyngeal nerves; V. 1 foramen by which the CTOOV6 in the orbito-nasal or first division of the fifth passes to the i 1 n /TV nasal cavity dry skull (Fig. 4, Ep.} These ossifications, therefore, answer primarily to the exoccipitals, but, in addition, represent the. opisthotic and epiotic elements. Above the fenestra ovalis, the wall of the otic capsule is produced out wards into a stout shelf, which forms the roof of the tympanic cavity, and corresponds with the tegmen tymr I- 95 Fig. 5. ,.JT FIGS. 4, 5, G, 7. Dorsal, ventral, lateral, and posterior views of the skull of Rana escuknta. The letters have the sama signification throughout. Pmx. premaxilla ; MX. maxilla ; Vo. vomer; Net. nasal; S.e. sphen-ethmoid; ft: frontal; Pa. parietal; E.o. cxoccipital; Ep. epiotic process; Pr.O, pro-otic; t.t. tegmen tympani; Sq. squamosal; QJ. quatl- rato-jugal; Pt. 1 pterygoid, anterior process; Pt? internal process; Pt? posterior or external process; C.a. columella auris; St. stapes; Zfy. hyoidean cornu; P.S. parasphenoid;