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depression, presents a rough convexity, bounded posteriorly by a transverse 

 ridge of the par-occipital, instead of the hemispherical depression for the arti- 

 culation of the stylo-hyoid bone, which characterises the skull of the Mylodon. 

 The basi-occipital (g) presents a median smooth concavity and two lateral rough 

 depressions which are continued on to the basi-sphenoid (A), and indicate the 

 insertions of very powerful ' recti capitis antici majores': the obliterated suture 

 between the basi-occipital and basi-sphenoid forms a rough transverse ridge : the 

 inequalities of this part of the basal region of the skull present a striking con- 

 trast to the broad, smooth and even tract which the same part forms in the 

 Mylodon*. The sides of the concave under surface of the basi-sphenoid are 

 bounded by longitudinal ridges, which have been broken off in the specimen. 

 The petrous bone terminates by a prismatic pointed process in the foramen 

 lacerum (i), which here gives passage both to the jugular vein and internal 

 carotid. The foramen ovale (k) is circular, and of the same size as the anterior 

 condyloid foramen. The foramen rotundum (/) is one inch and a half in 

 advance of the foramen ovale, and opens into the commencement of a deep and 

 long groove which traverses the base of the pterygoid processes in the direction 

 towards the ant-orbital foramen. The base of the zygomatic process supporting 

 the articulation of the lower jaw (m) is brought much nearer the occiput than 

 in the Mylodon, and is separated from the petro-mastoid by a deep excavation 

 perforated by wide apertures that seem to communicate with the tympanic 

 cavity. The articular surface for the lower jaw is well-defined, narrow in the 

 axis of the skull, much extended transversely, gently convex in both directions. 

 In the skull of a recent Armadillo {Dasypus octocinctus), the articulation for the 

 lower jaw is almost flat and on a level with the roof of the posterior perforated 

 cavity: in the Prionodon (Dasypi/s gigas, Guv.), the articular surface is slightly 

 concave and extends longitudinally forwards from the posterior cavity: the zy- 

 gomatic process of the malar bone bounds the outer and fore part of the surface, 

 and extends forwards in the form of a laterally compressed plate of bone, and 

 in the Das. sexmiclus forms a slight angular projection below the ant-orbital 

 perforation. In the Glyptodon the articulation for the lower jaw more resembles 

 that in the ordinary Pachyderms, and is thus conformable with the deviation 



* See Memoir on the Mylodon, 4to, pi. iv. 

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