26 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 
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with equal acumen has made it the basis (op. cit. p. 501) of determination of the 
isolated chevrons of Paleohatteria as median caudal. 
Seeking the explanation of this overarching in Sphenodon, it pondered 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 !. 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 Pl. 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 
vertebree and the fibro-cartilaginous interarticular masses of Sphenodon are organically 
continuous. 
We are constrained to do this as Cope more particularly ?, and Gadow, though with 
greater caution ? (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. cit. 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. cié. p. 173), 
who, from the study of the paleontology 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 preformed in paired cartilages (PI. 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). 
‘ We fully concur in Boulenger’s refutation (Ann. & Mag. Nat. Hist. ser. 6, yol. xii. 1893, p. 60) of Dollo’s 
hypothesis that “ heemapophyses are homologous in all Vertebrata.” 
* Cope, E. D.: Trans. Americ. Philos. Soc. vol. xvi. 1886, p. 248. 
° 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. 
