DEVELOPMENT OF THE SKELETON OF THE TUATARA. 31 
cartilaginous border with that of the second sacral rib. ‘This was stouter than usual, 
while the first rib, reduced externally to a degree proportionate to the extension of the 
first caudal into the sacrum, was reduced to the calibre of a presacral rib. The 
combined cartilaginous heads of the three ribs, by union, furnished the iliac articulation, 
and there can be no doubt that the determining cause of this abnormality was the 
backward rotation of the left ilium, well marked. It may be a case of ‘“ vertebral 
assimilation ’ !, but its interest is none the less if so. 
We have only to add, concerning the caudal region, that at Stage S, when ossification 
is setting in, the detailed relationships of the caudal rib to its centrum and arch are 
closely identical with those of the sacral vertebre (cf. Pl. II. figs. 5 & 6), and that at 
this stage comparison with the presternal region (fig. 4) shows that the interarticular 
tissue between the rib and its vertebra is far less differentiated than there. 
The most conspicuous difference recognizable on a survey of the whole series of rib- 
heads in the adult and the later developmental stages of Sphenodon, is the fact that 
while the individual rib is for the most part obliquely attached or articulated upon the 
anterior end of its related centrum, in the case of the third and fourth presternal ribs, 
which are two-headed, this antero-veutral extension is more marked and involves the 
intercentrum, the capitular head abutting against that. For this an explanation has to 
be sought. 
Examination of the posterior trunk, sacral, and tail vertebre, at the period of 
chondrification, would seem to indicate (of. figs.) that the ribs arise only in relation to 
the vertebral bodies, and that they may be derivative of either their arch system, as 
originally believed for the higher Vertebrata by Gegenbaur, Goette, and others ; or of 
their transverse processes, as argued by Claus, and more recently by Corning? and 
others—2. ¢., that in any case they would appear to be vertebral in origin, as has been 
more recently maintained for the ribs of the Vertebrata generally by Dollo *. Hofmann, 
in a memorable paper published in 1878 +4, from the study of both the anatomy and 
development of adequate representatives of all the amniote classes but birds, deduced 
the final conclusion that the ribs of these animals are primarily intervertebral. Baur 
has further sought to support this view °, and Gadow, among most recent investigators, 
has done likewise, in his final conclusion (96. p. 50) that the ribs of the Amniota are 
lateral outgrowths and the chevrons and hemapophyses ventral outgrowths of his 
‘**basiventralia,” which give rise to the intercentra °. 
' Cf. Siebenrock, F.: Ann, naturhist. Hofmus. Wien, Bd. vii. 1892, p. 373. 
* Corning, H. K.: Morph. Jahrb. Bd. xvii. 1891, p. 611; cf. also Goeppert, ibid. Bd. xxv. 1897, p. 247. 
% Dollo, L.: Bull. Sci. Fr. & Belg. tom, xxiv. 1892, p. 113. For other references, see Hofmann, infra. 
* Hofmann, C. K.: Niederld. Archiv, Bd. iv. 1878, p. 199. 
5 Baur, G.: Amer. Nat. vol. xxi. 1887, p. 942. 
° Psychologically interesting in this association is the conclusion of Dollo (op. ct. p. 128), who, though 
? 
apparently in error concerning the vertebral origin, regards the “ ribs ” as dorsal ribs, the ‘* hemapophyses © 
as ventral, 
