OF THE SKULL IN THE AMPHIBIA UKODELA. 195 



number. Proteus and Menobranclms have only three ; the 4th epibranchial is also 

 wanting here. Thus in several points these larva? approach the lowest " Peren- 

 nibranchs." 



Notwithstanding the large size of this larva, the flat 2nd basibranchial (h.br 2 ) is 

 very imperfectly differentiated from the 1st (b.br 1 ) ; as to form, these parts are quite 

 normal. 



Skull of larva of Spelerpes salmonoa, 2 inches 1 line m length. 



The metamorphosis of this type of skull has been as profitably followed in the larvae of 

 two species as if I had possessed all of them of the same kind. The head of the one 

 just described is broader, and more like that of Me nol ranch us ; in this species (PI. XX. 

 figs. 3-6) we have a nearer approach to the sharp eel-like head of Proteus. Therefore 

 also, as these are more advanced than the last, we have to deal with characters that are 

 due to metamorphism, and also with those that are due to specific difference. 



The notch between the occipital condyles (figs. 3, 4, oc.c) is shallower ; for the 1st or 

 odontoid and 2nd or atlantal vertebrae are much shorter than in the last species. 



Although more advanced, the supraoccipital region (so) is not yet ossified ; the 

 basioccipital is more so, and Ave see the very small remains of the cephalic notochord 

 (nc), so unlike what is to be observed in many Urodelous larva? at this stage. 



The occipital roof is of larger extent, and the auditory capsules are directed more 

 outwards in front, carrying with them the suspensoria, which thus give the larva? a 

 wider gape. 



The amount of cartilage seen on the front face of the capsule from above (fig. 3) is the 

 same as in the last ; but the vestibular shell has a more even and a narrower rim of 

 cartilage round the fenestra ovalis, all but its outer side (fig. 4). It will be seen in 

 these two embryos (figs. 3, 4, 5, & 6) that the " tegmen tympani " is more tilted upwards, 

 so that the fenestra and its lid, the stapes (st), are brought very close to the outer edge 

 (see also the difference in PI. XX. figs. 9, 10). 



Ossification is spreading from the centre to the circumference of this plate, a neat 

 margin of cartilage still remaining. 



This oval bony centre has two curious enlargements on its surface ; one is a boss on 

 its hinder half for the insertion of the muscular fibres that roughly represent the 

 stapedius muscle, whilst the other, on the fore half, is a neat round handle to this 

 vestibular lid (figs. 4 & 10). As the plate itself has a rim of cartilage, so this rod has a 

 core at its distal end ; hence it is evident that a distinct rod of cartilage has been 

 ensheathed by a hollow process from the stapedial bone, just as the prootic ensheaths 

 the trabecular in these larva? (see especially fig. 6) and in the adult Proteus. 



We shall see that this cartilage may have its own osseous centre (see PL XVIII. fig. 9, 

 in Desitioc/nathus, on the left side only); that is, however, when it does not grow 

 so close to the stapes. Now here, it would seem, we have at last found a veritable medio- 

 stapedial. Proteus has an unmetamorphosed hyomandibular ; and what more natural 

 than that these proteiform larva? should repeat that segment, and use it as a " colu- 

 mella"? 



