Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/803

Rh AMPHIBIA 759 lated with the styliform projection of the centre of the stapes. A veiy strong hyo-suspensorial ligament passes from near the distal end of the suspensorium to the cornu of the hyoid. The mandibulo-hyoid ligament is much weaker. The hyoidean apparatus presents a median basi-hyal, con nected by a rounded hypo-hyal on each side, with a long and curved cerato-hyal, which is almost completely ossified. The first basibranchial is elongated and cartilaginous the second is absent. The first branchial arch is a single elon gated bone, representing the similarly coalesced cerato- branchial and epibranchial in Menopoma. The second cerato-branchial is small and cartilaginous. The three posterior epibranchials are simple carved cartilages; and the single branchial cleft is placed between the third and fourth epibranchials. The skulls of the four genera, Mendbranchus, Proteus, Siren, and Amphiuma, now described, resemble one another, and differ from those of other Amphibia, in their elongated form; and, especially, in the relative narrowness of the facial region in front of the orbits, which, as the case of Amphiuma shows, arises, not from any want of development of the maxillary bones, when they exist, but from their taking a direction which but slightly diverges from paral lelism with the axis of the skull. Moreover, they all possess well-marked epiotic prominences. Amphiuma differs widely from the other three, in the great size of its maxil lary bones, in the absence of palatine bones, in the pro jection of the occipital condyles beyond the epiotic pro cesses, in the ankylosis of the premaxillse, in the presence of well-developed nasal bones, in the coalescence of the first cerato-branchial with the first epibranchial, and in the transverse direction of the suspensorium. In most of those respects, in which Amphiuma differs from Menobranchus, Proteus, and Siren, it approaches the Salamanders ; especially if we take such forms as Anaides into account. On the other hand, in the entire absence of a palatine bone, and in the fusion of the first cerato-bran chial with its epibranchial, it agrees with Menopoma and Cryptobranchus. In Menopoma, the skull has a broadly-rounded snout, and its posterior contour slopes forwards and outwards (with out being interrupted by conspicuous epiotic prominences), in the manner characteristic of the higher Urodela. The small pro-otics are separated from the exoccipitals (which also represent the epiotics and episthotics), by a wide car tilaginous interspace, in which the f enestra ovalis is situated. The parietal sends a process forwards, along the outer edge of the frontal, between it and the orbito-sphenoid. This meets a curved flat bone, which bounds the orbit anteriorly and internally, and articulates with an ascending process of the maxillary bone. It may therefore be regarded as a prefronto-lachrymal. The frontals unite in a long median suture, and then, diverging, embrace the nasal bones, and articulate externally with the fore part of the ascending process of the maxillary bone, which is thus received be tween the frontal and the prefronto-iachrymal. The very broad parasphenoid extends from the exoccipitals to the vorners, with which it unites by a denticulated squamous suture. The wide vomers are united by a median suture, and expand in front, ending in arched edges, close behind which the teeth are set. The premaxillse are separate and Email, articulate with the arched edges of the vomers. and send up strong ascending processes to the dorsal face of the skull, where they firmly unite with the nasals. The squamosal is a flattened prismatic bone, as broad at one end as at the other, which articulates with the parietal externally, and with the quadrate internally. Like the suspensorium, which it covers, it stands out at right angles with the axis of the skull. There is no palatine bone. The pterygoid is broader and more square than in any other Amphibian, in consequence of the great expansion of its internal process, which articulates by its whole length with the parasphenoid. The anterior process ends in a free pointed cartilage, directed outwards and forwards, and united with the maxilla by ligament, as in the higher Urodela. The external process extends to the articular end of the quadrate, as usual, and is continued thence along the cartilaginous suspensorium to its attached end. The chondrocranium forms a complete ring of cartilage round the occipital foramen, continuous at the sides with the auditory capsules. From these the trabeculaa are con tinued forwards, as in Menoliranchus, leaving a very wide ventral fontanelle. At the anterior end of this they unite and form the mesethmoid, from which roof and floor plates of the nasal capsules are continued. The suspensorium is connected by a pedicle with the trabecula, in front of the auditory capsule, and gives off a broad ascending process, which becomes ossified continuously with the pterygoid, over the orbito -nasal nerve. A stout otic process is articu lated with a facet on the antero-external region of the periotic capsule, and is further connected with it by liga- mentous fibres. The quadrate ossification involves a small portion of the articular end of the suspensorium ; it thence extends upwards, on the dorsal aspect of the suspensorium, gradually becoming more slender, and nearly reaches the point at which the otic process of the suspensorium articu lates with the periotic cartilage. The osseous skull of Cryptobranclms is extremely like that of Menopoma. In Menopoma the hyo-branchial apparatus presents the same general structure as that of Siredon, except that the second basibranchial seems to be wanting, while the first is very broad and rounded; at the same time, the epi branchial and the cerato-branchial of the first arch are represented by only one continuous cartilage. In Cryptobranchus,^ however, a considerable reduction has taken place, the two posterior pairs of branchial arches present in Menopoma having disappeared. The second arch still presents a division into cerate-branchial and epibranchial, but the dorsal end of the latter is closely united with that of the preceding arch. It is interesting to observe, however, that the modification thus effected ia quite different from that which occurs in the Salamanders, in which, in the adult state, the first branchial arch retains its two segments ; while the second, reduced to its cerato- branchial, is applied against the first, at the junction of the cerato- and epi-branchial; and the second basibranchial persists as the ossiculum thyroideum of Von Siebold. Menopoma and Cryptobranchus further differ from the proper Salamanders in having the vomerine teeth disposed along the anterior edges of the expanded vomers. Unfor tunately nothing is known of the larvae of these forms, but it would seem as if, in them, the primitive vomers enlarge by extension of ossification behind, and not in front of, the originally existent teeth. Tn the remaining Urodela, the Salamanders proper, the skull has the broadly-arched snout and the shelving posterior contour of Menopoma, but the vomers and ptcrygoids are very different. The structure of the skull in these animals will be best under stood by commencing with that of Siredon, which, though perenno- branchiate under ordinary circumstances, is totally unlike the other so-called Pcrennibranclviata iu cranial structure, and is, in fact, to all intents and purposes a larval Salamander. An ossification on each side of the occipital foramen represents the exoccipitals, epiotics, and episthotics. In front of each of these 1 See Hyrtl, &quot; Cryptobranchus japonicus, Sehediasma anatomicum,&quot; tab. iii.