DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 29 



can extend Ms discovery of an osseous rib for the third vertebra to an individual of 

 Dendy's Stage S (PI. II. fig. 1) and to adult specimens in the R. College of Surgeons 

 and British Museum. If his " ligamentous ribs " are really such, it is reasonable to 

 suspect that at some developmental stage they may be either definitely skeletogenous 

 or may show traces of their supposed skeletal origin. After careful search, we are only 

 able in support of this conclusion to point to the existence within the third ligament 

 of an individual of Stage S of an insignificant cartilaginous nodule. The fact that no 

 trace of a corresponding nodule was forthcoming in the two specimens of Stage E 

 examined, leads us to regard the afore-mentioned case as an individual one of the 

 presence of a vestigial rib. 



Concerning the relationships of these supposed ligamentous ribs, there is a detail 

 not recorded by Baur. As viewed at first sight (fig. 2), the double attachment would 

 seem suggestive of a two-headed rib ; but, in the case of the most distinctly two- 

 headed osseous rib (viz., that of the fourth prsesternal segment), the osseous capitulum 

 and the supposed ligamentous one coexist, while for both this rib and those behind it, 

 in which the capitulum is unrepresented in bone, the supposed ligamentous portion of 

 the rib, instead of passing into the substance of the osseous one, as should be the case 

 were it a constituent of the rib, merely skirts the lower border of that with an 

 accompanying attachment of its fibres. Throughout the region of insertion anteriorly 

 of the muscles termed by Osawa " costo-cervical," it leaves the rib and passes on as an 

 independent structure, in the manner delineated in PL II. fig. 2, m.' — in exactly the 

 manner which would result from an intimacy of mechanical relationship. When to 

 this is added the disproportionate development of the first of the so-called " liga- 

 mentous ribs," which is in reality for the most part a muscular mass, we doubt how 

 far these structures really represent ribs wholly or in part, and confess ourselves 

 unable to definitely decide the question either way^. 



Turning now to the rest of the prsesacral ribs, as concerning their heads and 

 articulations alone, we find as we pass back from the fifth cervical the distinction 

 between tuberculum and capitulum becomes gradually lost. With this simplification 

 there takes place a reduction of the transverse process, which, never strong, disappears 

 in the anterior sternal region, and, as pointed out by Osawa (98^ p. 735), is formed by 



' At the outset of om- investigation into the " cervical " region, mindful of the belief in the Cheloniau 

 affinity of Spheuodon and the assertion by W. K. Parker (' Challenger ' Reports, Zoology, vol. i. pp. 47 & 50) 

 that in the embryo of the Green Turtle the somatomes of the neck and tail are more numerous than later, and 

 undergo numerical reduction by abortion, we kept a look-out, in vain, for probable evidence of a similar 

 process, so far as it might involve tlie skeleton in Sphenodon, in the hope of being able to obtain some clue 

 to the meaning of the numerical variation of the reptilian prsesternal segments so conspicuous among the 

 Dolichosauria and Mosasauria. From the discovery of facts concerning the variation in relatiouship of the 

 sternum which we herein describe {infra, p. 34), we are disposed to regard numerical variation among the 

 prsesternal vertebrse, if not the poststernal also, as associated rather with a shifting of the sternum than an 

 excalation of segments, as Parker's allegation would seem to suggest. 



