124 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 



canal enters above the posterior cotyle and opens upon the side of the 

 vertebra ; the transverse processes are very small, hardly more than ridges 

 on the postero-external angles. 



The atlas is followed by a compound vertebra made up of the axis and 

 the three succeeding cervicals (Plate XXV, fig. i) completely fused to- 

 gether, but with lines of suture remaining on the ventral side of the 

 centra. The axis has a broad, much depressed centrum, low and broad 

 facets for the atlas and a short odontoid process, which is broad at the 

 base, but tapers rapidly to a blunt point; the neural canal is large and the 

 spine is a high, narrow, laterally compressed plate ; the vertebrarterial canal 

 opens in the base of the transverse process, immediately behind the anterior 

 cotyle. The other elements of the compound bone have very short and 

 broad centra, which are so much depressed as to be merely thin, transversely 

 arched plates ; the neural arches are quite high, but these vertebrae appear 

 to have no spines ; zygapophyses are visible only on the hinder part of 

 the fifth cervical ; the transverse processes of all the vertebrae are fused 

 together into a very long and heavy mass, which is thickened and rugose 

 at the distal end, and has three foramina on the ventral side. The sixth 

 cervical is free, but similar in form to the fifth, and in some specimens 

 the neural arch is reduced to a mere bony thread, and a very small spine 

 may or may not be present; the zygapophyses are large and are so 

 closely approximated that the anterior one extends over the posterior one 

 of the same side ; the transverse process is long and thin and extends 

 directly outward ; the anterior face of the centrum is transversely concave 

 and extends around the postero-external angles of the preceding vertebra, 

 quite as in Proeutatus. 



As in Glyptodon, the last cervical is fused with the first two thoracics ; 

 it has a longer and somewhat wider centrum than the sixth, with facets 

 for the first pair of ribs ; the neural arch is extremely slender and the 

 spine is absent. 



The number of thoracic vertebras appears to be eleven, of which the 

 first two are coalesced with the last cervical, the third, fourth and some- 

 times the fifth, are free, while the remaining six coalesce with each other 

 and with the first lumbar to form the "dorsal tube" (Plate XXV, fig. 2). 

 The first and second thoracics have centra which are short, broad and 

 depressed, though less so than those of the cervicals ; the second is quite 

 opisthoccelous ; the compound neural spine is low, but broad, and much 



