DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 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 prsesacral 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 vertebrae {cf. PL II. figs. 5 & 6), and that at 

 this stage comparison with the proesternal 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 prsesternal ribs, 

 which are two-headed, this antero-ventral extension is more marked and involves the 

 interceutrum, the capitular head abutting against that. For this an explanation has to 

 be sought. 



Examination of the posterior trunk, sacral, and tail vertebrte, at the period of 

 chondrification, would seem to indicate {cf. 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 — ^. e., 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 1 878 *, 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 5, 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 heemapophyses ventral outgrowths of his 

 " basiventralia," which give rise to the intercentra ^. 



1 Cf. Siebenrock, F. : A.iin. naturhist. Hofmus. Wien, Bd. vii. 1892, p. 373. 



" Corning, H. K. : Morph. Jahrb. Bd. xvii. 1891, p. 611 ; ef. also Goeppert, ibid. Ed. xxv. 1897, p. 247. 



3 Dollo, L. : Bull. Sci. Fr. & Belg. torn. xxiv. 1892, p. 113. For other references, see Hofmann, infra. 



' Hofmann, C. K. : Niederld. Archiv, Bd. iv. 1878, p. 199. 



° Baur, G. : Amer. Nat. vol. xxi. 1887, p. 942. 



° Psychologically interesting in this association is the conclusion of Dollo (op. cit. p. 128), who, though 

 apparently in error concerning the vertebral origin, regards the " ribs " as dorsal ribs, the " heemapophyses ' 

 as ventral. 



