MEGATHERIOIDS. 343 



to the plane of the hard dentine, parallel to each other, with a slightly 

 undulating course, having regular interspaces, equal to one diameter 

 and a half of their own area, generally anastomosing in pairs by 

 a loop, (a, dy) the convexity of which is turned towards the origin 

 of the tubes of the hard dentine, forming a continuous reflected canal. 

 The loops are situated near, and for the most part close, to the 

 hard dentine. In a few places one of the medullary canals may 

 be observed to extend across the hard dentine, and to anastomose 

 with a corresponding canal in the cement. The interspaces of the 

 medullary canals of the vascular dentine are principally occupied 

 by calcigerous tubes, which have an irregular course, form reticulate 

 anastomoses, and terminate in very minute cells, at least one hun- 

 dred times smaller than the calcigerous radiated cells of the cement. 



The more regular and parallel calcigerous tubes, which tra- 

 verse the thin layer of unvascular dentine, are given off from the 

 convexity of the terminal loops of the medullary canals. The 

 course of these tubes is more directly transverse to the axis of 

 the tooth than is that of the medullary canals from which they are 

 continued. They run parallel with each other, but with fine 

 undulations throughout their course. They have a diameter of 

 ^^\.h. of an inch, and have interspaces of about twice that diameter. 

 As the calcigerous tubes approach the cement, they divide and 

 subdivide, and become more wavy and irregular : their terminal 

 branches take on a bent direction and form anastomoses, dilate 

 into small cells, and many are seen to become continuous with 

 the radiating tubes of the cells of the contiguous cement. The den- 

 tinal cells are less distinctly defined than in the Mylodon, and of a 

 more elliptical form : their average diameter is g^^th of an inch, and 

 they are traversed by from nine to twelve of the dentinal tubes. 



The cement, which enters so largely into the composition of 

 the grinders of the Megatherium, is characterized in that extinct 

 animal by the size, number, and regularity of the vascular or 

 medullary canals which traverse it. They present the diameter 

 of i^oth of an inch, are separated by intervals equal to from 

 four to six of their own diameters, commencing at the outer 

 surface of the cement, they traverse it in a direction slightly 

 inclined from the transverse axis towards the crown of the tooth 



