DEVELOPMENT OF THE SKELETON OF THE TUATARA. 29 
can extend his discovery of an osseous rib for the third vertebra to an individual of 
Dendy’s Stage S (PI. Il. 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 R 
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 przsternal 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 PI. 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 presacral 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 
1 At the outset of our investigation into the “cervical” region, mindful of the belief in the Chelonian 
affinity of Sphenodon 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 inyolve the skeleton in Sphenodon, in the hope of being able to obtain some clue 
to the meaning of the numerical variation of the reptilian presternal segments so conspicuous among the 
Dolichosauria and Mosasauria. From the discoyery of facts concerning the variation in relationship of the 
sternum which we herein describe (infra, p. 34), we are disposed to regard numerical variation among the 
presternal vertebrae, 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. 
