22 PROF. G. B. HOWES AND MR. IT. H. SWINNERTON ON THE 



therefore be easily traced and seen to be continuf)us along the whole length of the- 

 vertebral column. We liave searched in vain for evidence of the " occasional breaking- 

 down " of the cuticular sheath (el. interna) in the vertebral regions as supposed by 

 Zykofffor .Siredon (Bull. Soc. Nat. Moscou, Bd. vii. 1893, p. 34). 



The main cells of the chorda do not undergo any marked change, but in the centre 

 of each interseptal mass, at the point coincident with that of greatest flexibility, 

 there has now arisen, apparently by accumulation of a fluid contents, a conspicuous 

 ^lobular vesicle (PI. I. fig. 15, v.c), which we propose to term the vesicula centralis, 

 and assume to be in some way associated with facilitating the movements of the 

 parts. 



Goette, by an ingenious argument, has drawn the conclusion {op. cit. p. 366), 

 primarily from the study of the caudal region, that in Sphenodon the vertebrse are not 

 truly amphicoelous. He extends this conclusion with qualification to the trunk- 

 vertebrse, which he figures as transmitting an uninterrupted notochord. Osawa has 

 adopted his view and applied it to the vertebral column generally (98*. p. 104). 

 We defer consideration of the caudal region, which is specialized on its own lines, till 

 later, but acceptation of Goette's view nevertheless depends entirely upon what we are 

 to understand by an amphicoelous vertebra. As constituted in many bony fishes, this 

 type of vertebra, at first sight solid, at its central and most completely constricted 

 region, is in reality perforated by a minute aperture or canalis dicentralis, for transmission 

 of a delicate thread-like process of the notochord connecting the adjacent inter- 

 vertebral masses. If such be termed an amphicoelous vertebra, as is customary, the 

 mere continuity of this notochord in Sphenodon would be no justification for dis- 

 regarding the accustomed terminology under which its vertebrae are described as 

 amphicoelous. If, on the other hand, by an amphicoelous vei'tebra be meant one 

 centrally solid and with excavated extremities, — i. e., a vertebra of which one or both 

 articular faces have not become condylar or flattened — then the term may be applied to 

 Sphenodon, the peculiar characters of its vertebrae being in this case the great depth 

 of its concavities. Inasmuch as we have shown the chordal plates to be notochordal 

 derivatives, we agree with Goette, in a sense, as to the presence of a continuous 

 notochord, which on the whole simplifies the customary conception of the Rhyncho- 

 cephalian vertebra. If, however, for argument's sake we dismiss the chordal plate 

 with the rest of the chorda, the characters of the Sphenodon-vertebra become more 

 nearly comparable to those of the cylindroidal type to which Credner has specially 

 drawn attention in Palaeohatteria {op. cit. p. 492), which may or may not have been 

 plate-bearing. Viewed from this standpoint, the extension of Goette's argument by 

 Osawa, instead of supporting his defence of the supposed Agamid affinities of 

 Sphenodon, weakens it to an unexpected degree. 



Passing now to the caudal region, the most important facts concerning the 



