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 XXXI. fig. 1) they have the general form and relations which 

 they exhibit in all known Salamandrine larvae ; and, so long as Sire- 

 don retains its branchiae, no important change takes place ; but in 

 the abranchiate Siredon (Amblystoma) and in Amblystoma Carolines 

 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 ex- 

 tends 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 ex- 

 tremities of the trabecute 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 trabecules 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 trabecute ; but it is inferior to the chondrocranium of 

 Siredon and of the ordinary Salamanders in the absence of subnasal 

 and supranasal ate, 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 posi- 

 tion 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. 



