258 GEOLOGICAL MEMOIRS. 



upwards and backwards 20° beyond the vertical line. The anterior 

 condyloid foramen, though large, is relatively smaller than in the 

 Mylodon, and is situated close to the anterior border of the condyle. 

 The depression for the digastric muscle is perforated and separated 

 from the condyle by a wider tract of the par-occipital than in the 

 Mylodon, and the petro-mastoid below the digastric depression 

 presents a rough convexity, bounded posteriorly by a transverse 

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

 for the articulation of the stylo-hyoid bone, which characterises 

 the skull of the Mylodon. The basi-occipital presents a median 

 smooth concavity and two lateral rough depressions which are 

 continued on to the basi-sphenoid, 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 contrast 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, which here gives passage both to the jugular 

 vein and internal carotid. The foramen ovale 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 pro- 

 cess supporting the articulation of the lower jaw 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 direc- 

 tions. In the skull of a recent Armadillo {JDasypus 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 

 (Dasypus gig as Cuv.), the articular surface is slightly concave 

 and extends longitudinally forwards from the posterior cavity : 

 the zygomatic 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. sexcinctus 

 forms a slight angular projection below the ant-orbital perforation. 

 In the Grlyptodon the articulation for the lower jaw more resembles 

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

 deviation from the Edentate structure manifested by the bones of 

 the foot. But the most remarkable characteristic of the skull of 

 the Glyptodon, by which it differs from the existing Armadillos 

 and approaches the Megatherioids, is the long and strong process 



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



