an inch in the short diameter. The cement is traversed by 

 numerous fine calcigerous tubes continued in many parts from 

 those of the hard dentine, but having a less regular course, which 

 is however generally at right angles to the surface : they terminate 

 principally in minute branches, which form a rich plexus around 

 the calcigerous cells with which they communicate. 



Fig. 2. A portion of the hard dentine of a tooth of the Mylodon rohustus, 

 as viewed with a power of 250 linear dimensions, showing the terminal 

 loops of the medullary canals of the soft dentine at a, and the fascicu- 

 late appearance which the minute calcigerous tubes present when 

 decomposition and carbonization have proceeded to a certain extent. 



Fig. 3. A thin transverse slice of a tooth of the Mylodon robustus, as 

 viewed with a power of 400 linear dimensions, including the hard den- 

 tine b, with a portion of the vascular dentine a, and a portion of the 

 cement c. These three substances enter into the composition of the 

 teeth of the Mylodon in the same relative position, and nearly in the 

 same proportions as in the Sloth : the layer of hard dentine is rather 

 thicker relatively to the other constituents. The central axis of vas- 

 cular dentine differs from that in the Sloth by the anastomoses of the 

 medullary canals by loops, near the hard dentine, towards which their 

 convexities are directed, as in the figure. The cement differs from 

 that in the Sloth by being traversed by vascular canals (rf), which are 

 directed towards the outer surface of the hard dentine, near which 

 they are most conspicuous : they are fewer in number than the cor- 

 responding canals of the vascular dentine, and do not form loops. The 

 radiated calcigerous cells are as numerous as in the cement of the Sloth, 

 but have a rather more elongated form, with the long axis parallel with 

 that of the tooth. The cement in the Mylodon is likewise traversed 

 by numerous and close-set calcigerous tubes continued from those of 

 the dentine, with a general direction transversely to the surface of the 

 cement, but with a more wavy and less parallel course, with more 

 frequent bifurcations and more numerous branches, the tortuous sub- 

 divisions of which open in great numbers into the calcigerous cells. 



