Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/804

Rh 760 AMPHIBIA Is a pro-otic, and an orbito-sphenoid. The latter is sometimes united with its fellow of the opposite side. 1 The skull is roofed in by pairs of parietal, frontal, prefronto-lachrymal, and nasal bones. The maxillaj are short, and are united with the ends of the suspen- sorium only by fibrous tissue. There is a very broad and flat para- sphenoid, which extends from the inferior margin of the occipital foramen, and ends, anteriorly, by a wide, irregularly convex edge, which does not reach the vomers. The latter bones are elongated and curved, and their long axes diverge posteriorly, as in Afcno- branchus ; but their anterior ends are far apart, and they lie, separ ated by the whole breadth of the parasphenoid, and between that bone and the premaxilla and maxilla, adherent to the ventral face of the subnasal process of the chondrocranium. The short palatine bones are situated immediately behind, and on the inner side of, the posterior nares, but their somewhat taper ing, external and posterior, ends do not articulate directly with the pterygoids. The latter are triradiate bones, with an inner process which passes towards the base of the skull ; an outer, which runs down the suspensorium ; and a long anterior process, which gradu ally diminishes in breadth forwards, and is connected only by ligament with the palatine. Three ossifications embrace Meekel s cartilage. The dcntary covers its outer face throughout its whole length. The angular lies on the inner face of its posterior two- thirds, and the small dentigerous splenial is also applied to its inner face between the angular and the dentary. The chondro cranium is in much the same condition as that of Menopoma. There is a broad basicranial cartilage situated between the auditory cap sules, and passing, at the sides and above, into a complete occi pital arch. But, in front, the trabeculas, though they have in creased in vertical height, remain united by fibrous tissue only, both in the floor and in the roof of the skull, which thus presents two great &quot; fontanelles &quot; when the parasphenoid, parietals, and frontals are removed. In front, they coalesce, each giving off, as it does so, a flat antorbital process, which is expanded at its outer end, where it supports the maxilla. Below, this process gives at tachment to the palatine. By their coalescence, the trabeculse give rise to a broad iuternasal septum (or mesethmoid cartilage), and they expand, on each side, below, into subnasal plates, which are separated, anteriorly, by a wide notch in the middle line. The curved outer edges of these plates give attachment to the premaxillse and maxillse, and they answer to the prse-nasal processes of the chondrocranium of the frog. Between the posterior edge of each of these and the anterior edge of the corresponding antorbital pro cess, the posterior nostril is situated ; and the inferior surface of the subnasal plate gives attachment to the vomer. Superiorly, the mesethmoid cartilage expands into a very thin (alinasal) plate, which roofs in each nasal chamber, and supports the prefronto- lachrymal and nasal bones. The suspensorium is connected, above and internally, with the trabecula of its side by a pedicle ; and it has an ascending process which lies over the orbito-nasal nerve (which is therefore included between the pedicle and the ascending process) immediately after its exit from the skull. Posteriorly and superiorly, it gives off an otic process, which is articulated with the outer and front part of the auditory capsule ; while, inferiorly and externally, it furnishes an articular surface to the mandible. The pterygoid process lias the form of a style tapering forwards, and nearly reaching the ant orbital process, with which it is connected, however, only by liga- mentous fibres. The posterior moiety of Meekel s cartilage is very stout as far as the coronoid process, and then tapers rapidly to its free, pointed, symphysial extremity. The hyoidean aud branchial apparatus is entirely fibrous and cartilaginous, none of its parts having undergone ossification. Each cornu of the hyoid is connected with the upper and posterior face of the suspensorium, and with the angle of the mandible, by liga- mentous fibres the hyosuspensorial and mandibulo-suspensorial ligaments. The cornua are not subdivided, and are united in the median ventral line by ligament. A triangular first basibranchial extends back from their junction, and is succeeded by a second, as in Menolranchus ; but this second basibranchial is not ossified. Two cerato-branchials are attached to the posterior extremity of the first basibranchial on each side, and the anterior is, as usual, fol lowed by a long and strong epibranchial, which supports the an terior gill. The posterior cerato-branchial supports the second epibranchial directly, and the third and fourth epibranchials in directly. The interesting observations of Professor 4 Dume~ril have shown that, under certain conditions, the ordinarily pcrenni-branchiate Sircdon passes into the caduci-branchiate Amblysloma, ; and this metamorphosis is accompanied by some very interesting modifica tions in the structure of the cranium, especially in the vomerine, palatine, and pterygoid regions. Ossification extends forwards from 1 See Friedrich and Gegenbaur &quot; Der Schadel des Axolotl (Sircdon pisdformis)&quot; in the Kerichte der Kuniylichen Zootomischen Ansta.il sra Wurzburg, 1849. This memoir contains an excellent account of the chondrocranium of the Axolotl. the vomers beneath the pne-nasal processes, so that tlie series of teeth, which originally lay along the anterior margins of these bones, come to be situated at their posterior edges. At the same time they take up a direction at right angles to the axis of the skull, instead of being greatly inclined to that axis, as they are in Sircdon, and the two sets of vomerine teeth thus form a single transverse row. Moreover, the anterior process of the pterygoid moves out wards until it comes into contact with the inner face of the maxilla. The one end of the palatine remaining attached to the vomer, the other swings outwards, in correspondence with the change of posi tion of the pterygoid, and thus becomes directed transversely to the axis of the skull, immediately behind the posterior nostril, its teeth continuing the transverse line of the teeth of the vomers. Sala manders with the teeth thus disposed have been termed &quot;lechrio- dont.&quot; The maxillary bones are larger than in Sircdon, but the jugal arch remains ligamentous. The dorsal ends of the cornua of the hyoid retain their ligamentous connection with the suspen sorium, and the ventral ends with the anterior basibranchial. The first cerato-branchial and epibranchial persist, and retain their articulation with one another. The second cerato-branchial re mains, but its dorsal or outer end becomes attached to the preced ing, and all the three posterior epibranchials disappear. The second basibranchial becomes detached as a Y-shaped piece, which lies in the middle line, in front of the larynx. In all the other Salamanders, the vomers, in the adult, present the same enlargement of the part in front of the teeth, at the ex pense of the region behind them, as in Amblystoma. But the teeth rarely offer the same disposition. More commonly they form two series, inclined to one another at a more or less acute angle, open forwards, and supported upon bony plates, which appear like pro longations of the vomers, extending backwards on the ventral face of the parasphenoid. This &quot;mecodont&quot; arrangement is strikingly exemplified by Salamandra maculosa, and still more remarkably by Plcthodon and Anaides, where these longitudinal series of teeth beneath the parasphenoid are commonly termed &quot;sphenoidal&quot; teeth. Duges J and other observers, however, have shown that, in larval Salamandrcc and Tritoncs, the vomerine and palato-pterygoid apparatus have, at first, the same disposition as in Sircdon; and l)uges has described the process by which the palatine bones, be coming detached from the pterygoids, which rotate outwards, anky- lose with the vomers, taking up a position beneath the parasphe noid, anil more or less parallel with the axis of the skull ; and it can hardly be doubted that the so-called &quot;sphenoidal&quot; dentigerous plates of other genera of Salamandrida have the same origin. If this conclusion be correct, it indicates a very curious morphological difference between the &quot;mecodont&quot; and &quot; lechriodont &quot; Sahtman- drida. In all the Salamandrida the parietal bones send long processes forwards on each side of the frontals. The parasphenoid is a broad flat plate. Very often the premaxillae are ankylosed into one bone, and the bones of the periotic capsule coalesce. In some cases there are epiotic processes or ridges. Maxillae are always present, and the snout is usually broadly arched. Nasal bones, distinct from the prefronto-lachrymals, are usually present. In the genus Anaides the skull is comparatively long and nar row, and the muzzle is less arched than usual. The single pre maxilla, and the two well-developed maxillae, follow the semicir cular curve of the broad subnasal plates, to the edges of which they are attached. The hinder free extremities of the maxilla; are curved upwards, and the jugal arch is represented only by ligament. Thus far the skull is salamandrine ; as it is also in the presence of dis tinct nasal and prefronto-lachrymal bones, in the disposition of the vomerine and so-called &quot;sphenoidal&quot; teeth, in the absence of an apparent palatine bone, and in the manner in which the pterygoid is produced into a long process, which becomes connected with tho inner face of the maxilla. But, in the well-marked downward and forward inclination of the suspensorium, and in the strong crests into which the epiotic processes are developed, the skull of Anaides is very like that of Siren. In the skull of Epicrium glutinosum (Fig. 14), which may be selected as an example of the PKHOMELA, the strong occipital con- dyles are continued into two ossifications, which rise on to the roof of the skull, where they unite in a short suture, and, spreading out so as to embrace the parietals, are continued over the auditory apparatus, as far as the squamosal and the quadrate bones. Vent- rally, no indication of any suture between these bones nnd the broad parasphenoid is visible ; laterally, they pass forward into a con tinuous ossification, which constitutes the side walls of the auditory capsule, and, in front of this, is perforated by the wide foramen for the trigeminal nerve, and enters largely into the lateral wall of the cranial cavity. The parietal bone rests on the dorsal edge of this lateral ossification, which terminates, anteriorly, by an irregularly 3 Recherches, pp. 172, 173, pi. xiv. fig. 89. Rusconi, Observations Anatomiqucs sur la Sirtiie, pi. vi. figs. 3 and 10.