EDENTATA. 279 



not merely in South America, but also in the northern half of 

 the continent as far as Kentucky. Some of the earliest genera 

 exhibit a curious oblique suture across each ramus of the 

 mandible; and they must have walked less habitually on the 

 outer side of their feet than the later forms. It is also to be 

 noted that the latest species are the most bulky. Hapalops, 

 Hyperleptus, and other genera are known by fragmentary 

 skeletons from the Santa Cruz Formation (supposed Miocene) 

 of Patagonia ; Megatherium, Mylodon, Scelidotherium, and 

 Megalonyx are genera of comparatively recent date. 



FIG. 160. 



Megatherium americanum; mandible, oral aspect, about one-eighth nat. size. 

 Pampa Formation (Pleistocene) ; Buenos Aires, Argentina, d, hinder end 

 of symphysis, between the molar teeth. 



Megatherium (tig. 160). This is the largest known Edentate, the 

 typical species, Megatherium americanum, measuring between five and 

 six metres in length, the tail alone being about one and a half metres 

 long. The head is comparatively small, with the mandible very deep 

 in the middle of each ramus and its symphysis produced into a narrow 

 toothless spout. The zygomatic arch is complete, with a descending 

 process from the jugal bone. The teeth form a close grinding series, 

 five in the upper, and four in the lower jaw. They are quadrate in 

 shape and extraordinarily deep, each growing from a persistent pulp 

 .mil exhibiting two transverse ridges on the crown, which are due to 

 the development of two relatively hard laminae of dentine. The vertebrae 

 are proved to be seven in number in the cervical region, and these are 

 followed by sixteen dorsals, three lumbars, five sacrals, and nearly twenty 

 caudal vertebrae. The neural spines are all inclined backwards, and 

 accessory zygapophyses are well-developed in the lumbar region. The 

 caudal region is very massive, most of the vertebra) bearing large 

 transverse processes, and at least 10 or 12 having chevron bones below. 

 The foremost chevron consists of two separate halves, the others are 



