Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/802

Rh 758 AMPHIBIA external to and below, the latter internal to, Meckel s cartilage. The skull of Proteus is, in its general characters, similar to that of Menobranchus, but is more extensively ossified. In Siren, the skull, &quot;which has the same elongated form and forwardly directed suspensoria as in Menobranchus and Proteus, possesses, in the epiotic region, two strong crests, which project backwards beyond the level of the occipital condyles, but are otherwise very different from the epiotics of the latter genera. The &quot; cxoccipitals &quot; and prootics are completely fused together, even in half-grown specimens, a mere rim of cartilage being left around the f enestra ovalis. The lateral walls of the skull present ossi fications extending from the exits of the orbito-nasal nerves, forwards to the mesethmoid, or internasal portion of the trabeculse, and completely encircling the olfactory fora mina. But these ossifications remain distinct for a con siderable time, if they ever coalesce. Each, therefore, represents half of the sphen-ethmoid of the Frog. The flat and wide parasphenoid extends forwards to the space left by the divergence of the vomers. On the roof of the skull, each broad parietal sends forward a pro longation along the outer edge of the frontal, which reaches the antorbital process. The premaxillse have very long ascending processes, which lie upon the mesethmoid, and are received between the ends of the frontal bones. Between these ascending processes one or two elongated ossifica tions are situated. These were termed &quot;nasals&quot; by Cuvier, but their position does not accord with this determination. The horizontal, or oral, portions of the premaxillae, on the other hand, are very short and edentulous, each being coated by a thin plate of horny substance. The maxillae are rudi mentary or absent. The vomers are two, flat, oval, bony plates, the ventral aspect of which is beset with parallel rows of teeth set in obliquely-disposed curved lines, the con vexities of the curves being turned inwards and backwards. The vomers are in contact anteriorly, but diverge posteriorly. The posterior extremity of each abuts upon a plate of similar form, but much smaller, and bearing fewer series of teeth, which lies on the ventral side of the origin of the antorbital process, and represents the palatine bone. The aperture of the posterior nares lies just opposite, and ex ternal to, the junction of these two bones. The suspensorium is short, thick, and completely carti laginous. Dorsally and internally, it is attached by a stout pedicle to the trabecula in front of the auditory capsule, while its dorsal and posterior face lies against the trun cated anterior face of the pro-otic. The angle formed where this face joins the outer face represents the otic process of Jlfenobranchus. There is no pterygoid process, nor any trace of a pterygoid bone. Just above the suspensorium, and seemingly connected with it, there proceeds from the anterior face of the pro-otic region of the skull, a strong, triangular, fonvardly-directed cartilaginous process. From the free anterior end of this, a band of fibrous tissue passes, and, encircling the eye, is attached to the antorbital process. The squamosal is a slender curved bone, extending from the epiotic ridge to the articular end of the suspensorium, where it is widest. It exhibits only a rudiment of the well-marked process which extends towards the stapes in Menobranchus and Proteus. The mandible presents a dentary, an angular, and a dentigerous splenial element; and the proximal end of Meckel s cartilage is ossified, giving rise to a dense nodular articulare. The dentary is tooth less, and supports the inferior horny beak. The cornu of the hyoid is very stout, and its ventral moiety is ossified. The much thicker dorsal moiety is cartilaginous, and its recurved dorsal end extends beyond the extremity of the skull. At a considerable distance below its apex, a strong short ligament proceeds from ita anterior face to the stapes. A broad sheet of ligamentoua fibres further unites the hyoid with the lateral walls of the ear capsule, and with the posterior face of the suspen sorium (hyo-suspensorial ligament) ; and a slender ligament (mandibulo-hyoid) proceeds from near the insertion of the hyo-stapedial ligament to the angle of the mandible. There are two basibranchials, both ossified, the posterior ending in short radiating processes; two cerato-branchials and four epibr anchials. In Amphiuma, the suspensoria are very little inclined forwards, and their long axes make nearly a right angle with that of the skull. The portion of the skull which lies behind a line joining the articular ends of the ossa quadrata is very much shorter than the region in front of it. Moreover, although the epiotic processes are pro minent, the occipital condyles project far beyond them. One bone represents the exoccipital, epiotic, and opis- thotic on each side. The pro-otic is large, and gives rise to the anterior moiety of a strongly-marked temporal ridge. Its exposed surface presents two fossae, divided by a nearly vertical linear elevation. The large parietals form the posterior portion of the temporal ridge, and diverge anteriorly, to be continued forwards, on each side of the frontals, to the sphcn-ethmoid. The frontals, in like man ner, diverge in front to receive a median ossification, Avhich is continuous with the coalesced median processes of the premaxillae. The anterior half of each frontal is rugose, as are the exposed surfaces of the sphen-ethmoid and of the nasal bones, and the integument is firmly adherent to these rugosities. The nasals are broad and triangular. The truncated base of each lies over the nasal aperture ; the inner edge articulates with the ascending process of the pre- maxilla; the outer edge joins first the maxilla, and then the sphen-ethmoid. The premaxillas are so thoroughly ankylosed that no trace of their primitive distinctness is to be seen. The large maxilla3 extend back for half the length of the skull, are firmly united with the adjacent bones, and are connected by dense ligament with the extremity of the quadrate bone. The greatly elongated vomers diverge but little ; nevertheless, they come in contact only by their anterior extremities. In the rest of their extent they are separated, in front, by a median ossification representing the anterior part of the sphen-ethmoid, and, behind, by the anterior forked prolongation of the parasphenoid which embraces this ossification. The rest of the parasphenoid is broad and flat ; it widens a little, in front of the auditory capsules, so as to form a rudiment of the &quot;guard&quot; in the frog s skull. The osseous pterygoid is a curved plate of bone, convex imvards and concave outwards, which articulates posteriorly with the quadrate, and, in front, stops short, at little more than half the distance from its posterior end to the internal nostril. The cartilaginous pterygoid process of the sus pensorium extends some way beyond it, and, widening, is attached by ligament to the maxilla. Posteriorly, the car tilaginous pterygoid is traceable, as a comparatively narrow band, on the inner side of the bony pterygoid, to the pedicle of the suspensorium, which is attached in front of the fenestra ovalis, and above the rudimentary &quot;guard&quot; of the parasphenoid. An ascending process passes from it be tween the orbito-nasal and the other divisions of the trigeminal. The otic process of the suspensorium, which is articulated with the outer face of the auditory capsule, is cartilaginous; but the rest of the suspensorium is ossified as a quadrate bone. This is, as usual, clamped to the skull by the squamosal, which is broad and expanded above, and narrow below. Behind, the suspensorium is directly articu-