196 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 1 7, 



quite so instructive in its bearing on that of Menobranchus as is the 

 skull of a Triton about the period at which it leaves the egg * (Plate 

 XXXI. figs. 2, 2a), or that of a larval Axolotl (Plate XXXT. fig. 1). 



Here the notochord occupies the centre of the future basis cranii, 

 terminating in front in a rounded apex. The parachordal tissue 

 exhibits no trace of chondrification ; but it is very interesting to 

 observe, on each side of it, the indication of an intermuscular septum, 

 separating two myotomes, and thus indicating, so far, a segmentation 

 of this region. 



The auditory capsules are spherical sacs, which lie, quite isolated, 

 on each side of" the notochord, at some distance from it, and are not 

 yet chondrified. The trabeculae abut against the notochord pos- 

 teriorly, but are wholly separate from any other structure. In front, 

 they have begun to coalesce and to give rise to the broad internasal 

 plate which is characteristic of the Salamandridea. On each side of 

 them are seen the nasal sacs, with the minute posterior nares opening 

 into the cavity of the mouth. Behind, and external to, each trabecula 

 is a stout cartilaginous rod, which obviously represents the sus- 

 pensorium ; but the dorsal end of this cartilage (m), though it lies 

 close to the trabecula, has not yet coalesced with it, and the mandi- 

 bular arch is therefore quite free. The external angle (o) corresponds 

 in its relation to the auditory capsule with the part similarly marked 

 in the Frog's tadpole and in Menobranchus. At (q) is the articular 

 surface for Meckel's cartilage (Mck) ; but neither "orbital," "ascend- 

 ing," nor " palato-pterygoid " processes are as yet developed. 



The hyoidean arch (Hy, figs. 2, 2 a) is as distinct and independent 

 as are the mandibular and trabecular arches ; it is an unjointed carti- 

 lage with a pointed dorsal end, which lies close to the auditory capsule. 

 At its ventral extremity it coalesces with its fellow ; while, behind, 

 it is continuous with a median cartilage, which represents the basi- 

 branchials and ends in a long spatuliform style. From the sides of 

 the median cartilage two ceratobranchials proceed, and are continued, 

 the anterior into the first epibranchial, the posterior into the three 

 other epibranchials. None of these parts are distinctly articulated, 

 the future joints being, at most, faintly indicated. Sundry ossifi- 

 cations are visible in the fibrous tissue contiguous to the cartilages ; 

 thus the dentary (D) and splenial (Spl) pieces of the mandible, 

 the squamosals (Sq), and the prsemaxillEe (Pmx) (already one bone) 

 have made their appearance. 



The vomers {Vo), each of which bears two teeth, lie far apart, on 

 the inner side of each nasal opening, and beneath the anterior end of 

 the trabecula. Behind these are two dentigerous ossifications of 

 the fibrous roof of the mouth, broad and rounded in front, but drawn 

 out behind into a sort of tail, which is directed towards the suspen- 

 sorium, though it does not reach the latter. These bones correspond 

 with the anterior moieties of the palato-pterygoids of Menobran- 



* The observations on which the following statements respecting Triton rest, 

 were made in 1858; but I did not publish them, as I could not then obtain the 

 materials for completing the history of the development of the Triton's skull, 

 Perhaps I shall be more fortunate this spring. 



