OWEN ON THE GLYPTODON. 259 



which descends from the base or origin of the zygomatic process 

 of the maxillary bone. This process is compressed, but in the 

 opposite direction to that in the Mylodon, viz. from before back- 

 wards, instead of from side to side : it measures five inches in 

 length from the ant-orbital perforation ; one inch and three-fourths 

 in breadth across the middle : the outer margin is entire, and as if 

 folded back ; the lower half of the inner margin is slightly notched, 

 the extremity of the process curves backwards. Both anterior and 

 posterior surfaces bear strong marks of the attachment of muscular 

 fibres. The small remaining portion of the maxillary bone on the 

 inner side of this process shows portions of three deep sockets of 

 the same diameter throughout, indicating the implantation of molar 

 teeth by a single excavated base ; and showing two longitudinal 

 ridges on both the outer and the inner side, which proves the 

 teeth to have had the same fluted exterior which they present in 

 the lower jaw, and of which the generic name of Glyptodon is 

 expressive. The fractured anterior part of the " basis cranii " 

 shows the large cavities for the olfactory bulbs and the remains of 

 a very extensive cribriform plate, the organ of smell being very 

 largely developed. 



The posterior or occipital surface of the skull slopes forward 

 from the plane of the occipital foramen at an angle of 45° : in the 

 small existing Armadillos it is vertical : in the Glyptodon it is 

 divided by a strong median vertical ridge, and separated by a 

 sinuous, thicker, transverse ridge from the upper surface of the 

 skull. The posterior half of this region of the cranium is marked 

 by the ridges bounding the origins of the temporal muscles, which 

 almost meet along the middle or sagittal line. Part of the lamb- 

 doidal suture is seen ; the other cranial sutures are obliterated. 

 The temporal fossae are pierced by numerous large vascular fora- 

 mina. The anterior parts of the temporal ridges diverge to the 

 posterior angles of the supra-orbital ridges. The frontal or inter- 

 orbital part of the upper surface of the cranium is broad, and 

 nearly flat, smooth and slightly concave at its posterior half, 

 slightly convex, rough and perforated by vascular foramina at its 

 anterior half. The most prominent parts, above the orbits, are 

 most rugose and indicate a more intimate adhesion to the super- 

 incumbent osseous dermal helmet. The lacrymal foramen is 

 pierced immediately in front of the anterior border of the orbit. 



The difference in the development of the temporal muscles 

 manifested by the Glyptodon and Mylodon, in the position of the 

 ridges on the fossil cranium, indicates a corresponding difference 

 in the power of mastication and in the density of the alimentary 

 substances habitually selected by each species : the greater pro- 

 portion of hard dentine in the teeth of the Glyptodon, and the 

 greater number of the teeth, which appears to have been thirty- 

 two, eight on each side of both jaws, coincide with the characters 

 of the cranium and support the inferences thence deducible. 



2. The Carapace. It is composed of thick, pentagonal ossicles 



s 2 



