Homologies of the Mammalian Accia^ 225 
garded as the centmm of the atlas,— a view which seems to 
be supported by its very large comparative size in the young 
condition, long before the anterior tubercle of the atlas 
makes its appearance, — then we must recognise in the odon- 
toid ligaments the terminal member of the series to which 
the transverse ligament of the atlas and the ligamenta con- 
jugal ia belong ; and indeed the arrangement of their fibres, 
some of which are continuous from side to side, is favourable 
to this supposition, and reminds one of the ligamentum con- 
jugale in the sheep. 
i 
Fig, 1. Atlas of a young Ghelonia virgata — after Kathke. a, The arch 
b, Osseous centre of the tubercle. 
Fig. 2. Dorsal vertebra of a young seal, for comparison with the following 
figures : a, Oblique process, h, Articular surface for head of rib. 
Fig. 3. Superior aspect of the axis of a human fcBtus. a, The centrum, 
6, Odontoid process, c, Superior articular surface, d, Bulging of the arch in 
the situation of the inferior articular surface. 
Fig. 4. Inferior aspect of the atlas of the same subject, a, Articular surface. 
Fig. 5. Cervical vertebra from the same subject, a, Oblique process, b. 
Part of the arch entering into the composition of the body of the vertebra. 
Note. — Since writing the above, my attention has been 
called to Eathke's work, " Ueber die Entwickelung der 
Schildkroten," in which (page 77) the view that the odon- 
toid process is the centrum of the atlas is strenuously urged, 
and strong evidence brought forward in its favour. See also 
Owen, " On the Homologies of the Vertebrate Skeleton,'' 
page 93. Eathke points out that the ligamentum suspenso- 
rium, which, in the birds and higher reptilia unites the 
odontoid process to the occipital condyle, is the serial repre- 
sentative of the intervertebral discs behind. He found that 
in most chelonians it consisted of true cartilage, and that 
in certain birds it was composed of fibro-cartilage. This 
view of the ligamentum suspensorium is quite consis- 
tent with the suggestion which I have offered, that the 
