''megathertoids. 337 



and are fewer in number. The radiated calcigerous cells are as 

 numerous as in the cement of the Sloth's molar, but offer more 

 conspicuously the elongated form ; their long axis being parallel, 

 as in the Sloth, with that of the tooth itself. They measure a^^th 

 of an inch in the long diameter. The cement, in the Mylodon, is 

 traversed likewise by numerous and close-set calcigerous tubes, 

 continued from those of the dentine, with a general direction trans- 

 versely to the surface of the cement ; but with a more wavy and 

 less parallel course, with more frequent bifurcations and more nu- 

 merous branches, the sub-divisions of which form a rich plexus 

 around each calcigerous cell. The dentinal cells are unusually con- 

 spicuous, presenting the appearance of a network in a thin section of 

 the hard dentine of these fossil teeth : they present a sub-hexagonal 

 form, and a diameter of i^th of an inch ; and are figured with the few 

 dispersed vascular canals of the hard dentine in Plate 79. 



Scelidotherium. — The genus Scelidotherium includes three known 

 species, which resemble the Mylodon more nearly than the Megalonyx, 

 but differ from both those genera, not only in the form of the teeth 

 but in that of the astragalus, and in the structure of the knee- 

 joint. The dental formula is the same as in the Sloth and Mylodon, 

 viz. J^4.(l) The sockets of the upper jaw are much closer together 

 than in the Mylodon, and the first is not separated by a wider 

 interval from the rest : they occupy an antero-posterior extent of 

 three inches, seven lines, in the Scelidotherium leptocephalum, and, 

 in the Mylodon robustus, one of five inches, four lines : yet the first 

 and second molars in the Scelidothere exceed those in the My- 

 lodon in size ; but the rest are of inferior size in the Scelidothere, 

 and the last is the least of those in the upper jaw, contrary to its 

 proportions in the Mylodon. The four teeth of the lower jaw are 

 also in closer order than in the Mylodon ; the length of the alveolar 

 series is three inches, ten lines in the Seel, leptocephalum, and five 

 inches in the Myl. robustus, but the longest transverse diameter 

 of the first tooth in the Scelidothere exceeds that in the Mylodon, 

 whilst that of the last tooth is half an inch shorter than in the 



(,1) Plate 80, figs. 1 and 2. The name Scelidotherium has been left out, by mistake, at the 

 foot of this plate, in reference to the above figures. 



Z 



