OCR Text |
Show 1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 197 chus, and they occupy the same position as the palatines of the higher Vertebrata. But these bones, in fact, undergo very singular changes of position in the Salamanders. In the young Siredon (Plate X X X I . fig. 1) they have the general form and relations which they exhibit in all known Salamandrine larvae ; and, so long as Siredon retains its branchiae, no important change takes place; but in the abranchiate Siredon (Amblystoma) and in Amblystoma Carolina dentigerous bones, obviously identical with these, occupy the position of the palatine bones of the Frogs, lying transversely to the axis of the skull immediately behind the posterior nares *. In the common Tritons and Salamanders, on the other hand, these bones, as Duges originally observed, gradually incline backwards and inwards parallel with the base of the skull, coalesce with the vomers, and become the long dentigerous tail-like prolongations of the vomers, which adhere to the under surface of the parasphenoid. It can hardly be doubted that the so-called "sphenoidal" teeth of Plethodon and other Salamandridea are of the same nature. Whatever direction the palato-dentary plates may take, however, they lose their primitive connexion with the pterygoid in all the Salamandridea, the anterior end of that bone and of its supporting cartilage moving outwards, and coming into connexion with the maxilla as it does in the Frogsf. The posterior nostril is, usually, bounded only by cartilage, or ligament, representing the antorbital process. As the development of the Triton advances, chondrification takes place in the base of the skull on each side of the notochord; it extends backwards, to give rise to the occipital condyles, upwards, to form the occipital arch, and inwards, to constitute the basioccipital region ; and the auditory capsules coalesce with the posterior extremities of the trabecular and with the parachordal cartilage thus developed. Moreover the internasal or mesethmoidal cartilage gives off expansions above and below the nasal sacs, which become the roofs and floors of the nasal chambers. But a large membranous fontanelle persists between the trabecular in the basi- and presphe-noidal regions; and it is only in the Frogs that this intertrabecular or "pituitary" space appears to become completely chondrified. Thus the chondrocranium of Menobranchus presents a little advance upon that of the larval Triton just leaving the egg, in so far as it possesses parachordal chondrifications, and in so far as the mandibular arch and the auditory capsules have coalesced with them and with the trabeculae ; but it is inferior to the chondrocranium of Siredon and of the ordinary Salamanders in the absence of subnasal and supranasal alae, and in the want of a complete cartilaginous occipital segment. No known Elasmobranch, Ganoid, or Teleostean fish presents so incompletely developed a chondrocranium as that of Menobranchus. * Profs. A. Dumeril and O. C. Marsh have already noticed the change of position of these bones in Axolotls which became metamorphosed into Amblystoma. t In the adult (branchiate) Siredon it is interesting to observe that the apex of the pterygoid cartilage already lies a little outside the outer margin of the palatine bone. |