Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/800

Rh 756 AMPHIBIA processes. Here they overlap a very singular bone, consist ing of two broad alie, which lie between the anterior edge of the frontal and the external nares, and of a median portion which is continued forwards, as a narrow, flat, curved process, between the nasal chambers, being received into a sort of groove of the chondrocranium. The bone is readily raised up from the subjacent chondrocranium, of which it appears to be quite independent. At the outer end of each of its aloe, and between the antorbital process and the nasal capsule, is a small, transversely elongated, slender bone, loosely connected by fibrous tissue with the foregoing. The ethmoid is completely cartilaginous. The parasphen- oid has the ordinary sword shape, except that the &quot; guard&quot; is extremely short; but its point extends along the base of the skull, passing between the nasal sacs, underlying their septum, and terminating close to the premaxillas. The vomers are represented by a transversely elongated rhomboidal osseous plate, devoid of teeth, which lies be tween the two posterior nasal apertures, and therefore much behind the anterior end of the parasphenoid. The side walls of the cranial cavity are ossified from the antor bital process to the anterior boundary of the foramen for the fifth nerve, just in front of which they are pierced by the optic foramen. There is no palatine bone. The pterygoid, in the main, resembles that of the ordinary frogs ; but, in consequence of the shortness and little back ward extension of the suspensorium, the outer process passes almost directly outwards, with hardly any back ward inclination. A bony plate, which extends backwards from the posterior edge of the inner and outer branches of the pterygoid, underlies the tympanic cavity and the audi tory capsule, and forms the floor of the Eustachian canal. The squamosal is a short broad bone, with a long anterior process, which becomes connected, by direct articulation, with the pterygoid, and by ligament with the maxilla. The premaxillse are small, and the maxillae are connected merely by ligaments with the suspensorium, there being no jugal. The columella auris is remarkably strong, and is bent in the middle, so that its two halves form an obtuse angle; the anterior half lies against the inner face of the tympanic membrane. The posterior half runs parallel with the posterior edge of the tegmen tympani, towards the fenestra ovalis. Ligamentous fibres fix the columella firmly, though movably, to the superior margin of the tympanic cavity, where it is bounded by the squamosal. The stout car tilaginous hyoidean cornua are attached just beneath the anterior and inferior part of the margin of the fenestra ovalis. The body of the hyoid is very small, but the two &quot;thyro-hyals&quot; are extremely long and broad cartilages. There is no ossified &quot;mento-Meckelian&quot; element. In Pipa, the skull is extraordinarily depressed and broad. The nasal bones are wide, flat, triangular, and quite distinct from one another, a forward prolongation of the coalesced fronto-parietals extending between the two. The parasphenoid, very broad in the greater part of its ex tent, and having the guard rudimentary, sends a narrow median process forwards underneath the nasal septum, as in Dactylethra. No trace of a vomer, or palatine bone, was to be found in the specimen examined. The pterygoid is very like that of Dactylethra, but its inner branch is greatly prolonged, and the floor sent under the Eustachian tube unites much more closely with the produced exoccipital. The squamosal is very small, and the place of its zygomatic process is taken by ligament. This ligament, however, unites with the pterygoid in the same way as the bony process which answers to it in Dactylethra. The columella is less massive than in Dactylethra, and the end which abuts against the tympanic membrane is imbedded in a disk of cartilage. The occipital condyles look outwards and back wards, instead of inwards and backwards, as in Dactylethra. The hyoidean cornua are wanting, the thyro-hyals being large, but not so large proportionally as in Dactylethra. The skulls of the UKODELA present a very interesting series of modifications, leading from a condition in which the cranium retains, throughout life, a strongly-marked embryonic character, up to a structure which closely ap proximates that found in the Amtra. In Menolranchus, for example, the chondrocranium of the adult is in nearly the same state as that in which it AA $ i ^r r r il^ & Fig. 12. FIGS. 10, 11, 12. Lateral, dorsal, and ventral views of the cranium of ifenc- branchus lateralis. In the dorsal view, the bones are removed from the left half of the skull; in the ventral view, the parasphenoid, palato-pterygoid, and vomers are given in outline. The letters have, for the most part, the samo signification as before. VII. p. posterior division of the seventh nerve ; VII. chorda tympani; V 1, V 2 , V 3 , first, second, and third divisions of the tri- geminal; t.s.l. stapedio-suspensorial ligament; h.s. I. hyo-suspensorial ligament; m. h. I mandibulo-hyoid ligament; a, ascending process of the suspensorium ; p, ptery go-palatine process; g, quadrate process; o, otic process ; .Aft. posterior nares ; Mfk. Meckel s cartilage ; Gl. (Fig. 10), the position of the glottis. lib 1. BI-, basibranchials. exists in a young tadpole or larval salamander (Figs. 10. 11, 12).