26 PEOF. G. B. HOWES AND ME. H. H. SWINNEETON ON THE 



with equal acumen has made it the basis (op. tit. p. 501) of determination of the 

 isolated chevrons of Palseohatteria as median caudal. 



Seeking the explanation of this overarching in Sphenodon, it occurred to us, having 

 proved the chevrons proper to be permanent primary intercentra, that their united 

 "" proximal ends," being serial with our secondary intercentra, may be the homologues of 

 those and that the anterior 4-5 chevons are therefore complex 1 . In support of this con- 

 clusion we would point out that at those stages at which the secondary intercentra are 

 undeveloped (PL I. fig. 12 and PI. II. fig. 13) the extremities of the chevrons in question 

 are in no way united ; and proof of its accuracy has come to us in a specimen belonging 

 to the Dublin Museum, in which (PI. I. fig. 21) the right half of the anterior chevron 

 {i.p-") and the overarching lobe (our secondary intercentrum) have remained distinct. 



We append, in tabular form (p. 27), a synopsis of the complicated series of changes 

 undergone by the successively formed sets of intercentra ; and in conclusion desire 

 once again to emphasize the fact that in the most fully differentiated state the bony 

 vertebrae and the fibro-cartilaginous interarticular masses of Sphenodon are organically 

 continuous. 



We are constrained to do this as Cope more particularly 2 , and Gadow, though with 

 greater caution 3 (reading as we believe theory and expectation into fact), have referred 

 to the intervertebral masses in terms applicable only to discontinuous skeletal parts. 



The so-called " Pro-atlas." The most recent view expressed as to the morphology of 

 these debateable elements is that of Gadow, who (op. tit. pp. 12, 13, and 37) has come 

 to regard them as parts of the atlas the serial homologues of his " supradorsalia " of 

 the supposed ancestrally composite vertebra. In the conclusion that the atlas was 

 thus originally more complex than has hitherto been supposed, and that the atlas and 

 " pro-atlas " represent one vertebra, he has been followed by Osborn (op. tit. p. 173), 

 who, from the study of the palaeontology of the Reptilia, has come to regard the atlas 

 as composed of five pieces and " persistently rhachitomous." Despite all attempts to 

 discover facts which might help further to elucidate this question, we have failed. 

 Appeal to the nerves availed us nothing, and all that we can add is that the " pro- 

 atlas " so-called is praeformed in paired cartilages (PL II. fig. 3,p.a.), which at Stage Q 

 arise independently of both the cranium and the rest of the vertebral column, and are 

 imbedded in the tendons of the dorsal skeletal muscles, near the point of attachment 

 of these to the exoccipital, and that their articulation upon the skull is secondarily 

 acquired (cf. Howes, 90, p. 357). 



1 We fully concur in Boulenger's refutation (Ann. & Mag. Nat. Hist. ser. 6, vol. xii. 1893, p. 60) of Dollo's 

 hypothesis that " hsemapophyses are homologous in all Yertebrata." 



2 Cope, E. D. : Trans. Americ. Philos. Soc. vol. xvi. 1886, p. 248. 



3 Gadow, H. : Phil. Trans, vol. 187 B. (1896), pp. 33 & 51. The term " intervertebral disks " is apt to 

 create the impression of existence of independent elements. 



