Homologies of the Mammalian Aocis y fyc. 225 



garded as the centrum 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- 

 jugalia 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- 

 jugate in the sheep. 



Fig. 1. Atlas of a young Chelonia virgata — after Eathke. 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, b, Articular surface for head of rib. 



Fig. 3. Superior aspect of the axis of a human foetus, a, The centrum, 

 b, 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 Bathke'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 



