1888.] MORPHOLOGY OF SUPERNUMERARY PHALANGES. 497 



it attains an adequate development as such, disposed ventrally (cf. Hyla 

 freycineti, p.s, Plate XXIV. fig. 3), and there is a constancy of rela- 

 tionship betweenitandtheadjacent head of the terminal phalanx(j9.^, of 

 fig. 3), the two generally coming to underlie, more or less completely, 

 the distal epiphysis of the penultimate phalanx. This is, moreover, 

 generally enlarged and invested in a fold of skin which projects freely 

 forwards in the manner of a prominent lip, the whole giving to the an- 

 tero-dorsal extremity of the digit a very characteristic aspect, to which 

 Boulenger's fig. 1 does full justice. On comparison with Nototrema 

 (fig. 7, *), in which the supernumerary phalanx attains but a small 

 development, it is seen that this fold (*, fig. 7) is wholly related to the 

 enlargement in question ^ ; and it will be found, in all cases, that 

 the parts are so dispo.-ed as to allow of an upward rotation of the 

 terminal phalanx. When the latter is fully displaced its long axis 

 is seen to lie at right angles to that of the penultimate phalanx. It 

 will be found on manipulation that the degree of ventral dis- 

 placement of the supernumerary phalanx is here proportionate to 

 that of the upward rotation of the terminal one, and that when the 

 extremity of the digit is in contact with an applied surface, these 

 two phalanges lie in the same plane, the former receiving, together 

 with the base of the latter, the more direct thrust under the weight 

 of the falling body. Such an arrangement would manifestly result 

 in a distinct functional advantage, especially in the platydactyle forms, 

 and the terminal phalanx would be the better able to support, 

 undisturbed, the adhesive integument. 



When examined microscopically, the supernumerary phalanx is 

 seen, in its fully differentiated condition, to consist in most cases of true 

 hyaline cartilage (ex. Hyla arborea, fig. I, and Rhacophorus, fig. 2), 

 differing in no respect from that forming the epi|)hysial extremities 

 of the adjacent phalanges. It remains in this condition long after 

 the other phalanges, including the terminal one, have become 

 ossified (cf. Rhacophorus). It invariably ossifies quite late; and 

 among the large series of specimens examined we have met with it 

 in the bony condition only in Hyla freycineti and Rhacophorus 

 maximus. In the former instance it is seen (fig. 3) to be replaced 

 in a true endostosis. 



It might appear from the foregoing that its first development 

 takes place subsequently to that of the other phalanges, and that its 

 ossification sets in at a period relatively proportionate to that of the 

 same. Examination of the tadpole of Hyla arborea (fig. 1) shows 

 that this is not the case, for it is there as fully differentiated as with 

 the adult, and that at a stage in which the periosteal growth of the 

 adjacent elements is dawning. Nor must it be imagined that its 

 conversion into bene is in any way determined by its relative size, for 

 in Rhacophorus eques {^g. 2,p.s.) it is, while still unossified, relatively 

 larger than in any species with which we have dealt. 



On passing from the ossified type to that of the other extreme of 

 the series (to those forms, that is, in the adults of which anything 



''■ In the larva the conditions are otherwise, cf. Hyla, fig. 1. 



