442 M. Series on Glyptodon ornatus. 



fourth and fifth. The names of trivertebral and pentaverte- 

 bral bones, evidently cannot serve to designate these two tetra- 

 vcrtebral bones ; for the sake of clearness and brevity I prefer to 

 adopt the denominations mesocervical bone and metacervical bone. 



The metacervical bone presents the general lineaments of the 

 trivertebral bone of G. clavipes. It is only a little longer in 

 proportion to its breadth. The two costal impressions are more 

 oblique. Of the four apertures of conjugation, the last three 

 present the same general arrangement as in G. clavipes, and the 

 first is immediately in front of the second. 



The articulation which unites the metacervical to the meso- 

 cervical bone is closer than in G. clavipes. Below, in particular, 

 it leaves no hiatus, even in its extreme movements. The juxta- 

 posed vertebral bodies, instead of being reduced to the condition 

 of a trenchant lamina, measure about five millimetres in diameter. 

 The line which separates them describes a curve with its con- 

 cavity in front. It is here the fifth cervical pair of nerves that 

 traverses the articulation. 



The mesocervical bone corresponds with the pentavertebral 

 bone ; it has all its general features, except that it includes only 

 four vertebras. The ala of the bone, resulting from the coa- 

 lescence of the transverse apophyses, is comparatively much 

 slenderer than in G. clavipes. On the outside it terminates in 

 a narrow, nearly triangular surface. The posterior face of the 

 bone does not exhibit any special aperture for the passage of 

 the vertebral artery. 



We have here, therefore, an anatomical structure very similar 

 to that of G. clavipes, but to a certain extent displaced, thrown 

 back one vertebra. In this arrangement we cannot, however, 

 see a teratological anomaly : the existence, in the collection of 

 of the Museum, of two identical specimens, does away with any 

 supposition of this kind. 



Does the bony constitution of the neck just described pertain 

 to Glyptodon ornatus ? Everything leads me to suppose so, but 

 the future alone can settle the question. 



I hoped to get some light from another dorsal column 

 derived from an individual similar in size to that of which we 

 have the carapace. But this dorsal column was accompanied 

 by the metacervical bone, of which it had been the continuation, 

 and this was a true trivertebral bone analogous to that of G. 

 clavipes. But on examining it more closely some differences 

 were found between this dorsal column and that of G. ornatus. 

 Thus the anterior extremity of the spinous crest instead of being 

 oblique, thin, aud trenchant, is five or six millimetres in thick- 

 ness, and rises vertically to a great height. The lateral chan- 

 nels are also narrower and deeper. 



