DEVELOPMENT OF THE SKELETON OF THE TUATARA. 61 



proportion to that of the corresponding centralia of the other limb (fig. 12). Bayer has 

 suggested that in the presence of the double centrale Sphenodon exhibits a Batrachian 

 tendency, drawing attention to the great extent to which the living Urodela are doubly 

 centralial. One of us has, with another, attempted to show l that the presence of a 

 second centrale carpi is a diagnostic character of the Anura, and this goes to support 

 Bayer's argument. As to centralia, however,Wiedersheim has demonstrated 2 the presence 

 of three in the Axolotl ; and with these facts in mind it occurred to us that the inequality 

 in development of the two centralia in Sphenodon might possibly be due to the presence 

 of a third, which had fused with the opposite fellow on the opposite sides in the pair 

 of limbs under discussion. At Stage Q the condition of the centralia was found to be 

 the same as in the normal adult; but in a young specimen belonging to the R. College 

 of Science, Dublin, the praeaxial centrale is on each side ossified from two independent 

 centres (PL VI. fig. 14, pr.c.), and in such a form that while co-ossification of them 

 would bring about the condition of the right limb (fig. 12), co-ossification of the 

 middle nucleus with the postaxial centrale (po.c.) would result in that of the left. 



Bruhl, for some unaccountable reason, systematically terms 3 the pisiform of Reptiles 

 the " ulnar sesamoid " ! Concerning the rest of the proximal carpals and the centralia 

 we have nothing to add. 



The distal carpals, stated by Giinther to be five in number, are by Bayer enumerated 

 as four. As he was dealing with a young animal he found only the 4th carpal ossified. 

 This is seen to be the case on the left side of our figs. 12 and 13, in which (Stage T) 

 the rest of the carpus is still cartilaginous (cf. previous statement on p. 60). Bayer 

 believed that the 5th carpale is to be sought in the cartilaginous head of the related 

 metacarpal (op. cit. p. 241) ; but in this he was mistaken, as pointed out by Baur 

 in that it is always present and distinct, though small (PL VI. figs. 11 to 14). 



We have no variations or other matters to record concerning the phalanges of the 

 fore limb. 



The Tarsus. Giinther describes two proximal and two distal elements, and is doubtful 

 about a fifth the fifth tarsal. He alludes without name to the " meniscus " of Born, 

 which we figure (PL VI. fig. 18, mn.). Osawa describes one proximal element, the 

 " proximal tarsale," and four distal five in all. He regards the meniscus (p. 504) as the 

 first tarsale, while Born associates it in the Lacertilia 4 with the centrale. Both are 

 in error, for whereas the meniscus does not appear till Stage S (when all the tarsal 

 elements have been formed), the first tarsal arises at Q (fig. 16) as a separate element 

 (I.), and at Stage R, when fully chondrified, unites with its metatarsal. The meniscus is 

 thus proved to be an accessory element (cf. Born, Morph. Jahrb. Bd. vi. 1880, p. 67). 



1 Howes & Ridewood : P. Z. S. 1888, p. 177. 



2 Wiedersheim, R. : Morph. Jahrb. Bd. vi. 1880, p. 581. 



3 Briihl, C. B. : Zootomie aller Thier Kl., Wien 1880, pis. 31-34 and 53 & 54. 



4 Born, G. : Morph. Jahrb. Bd. ii. 1876, p. 25. 



