336 MEGATHERIOIDS. 



sheath of hard unvascular dentine, about one line and a half in thick- 

 ness, with an exterior covering of cement, about half a line in thick- 

 ness. The vascular dentine is permeated, as in the Sloth, by long, 

 nearly straight and parallel vascular canals, proceeding, for the most 

 part outwards, and with a sHght incHnation, to the grinding surface ; 

 this inclination is least at the base, and greatest at the summit of the 

 tooth, where the vascular canals are parallel with the axis of the tooth. 

 They have a slightly undulating course ; a diameter of ^Joth of an 

 inch, which they preserve throughout with little variation ; and 

 have intervals of about twice that diameter. They differ chiefly 

 from the vascular canals in the Sloths, by anastomosing together 

 in regular loops, turned towards, and close to the inner surface 

 of the hard dentine. Very fine calcigerous tubes are given off 

 into the interspaces of the vascular canals, from every part of their 

 course; but the most abundant penicilli of these tubes proceed 

 from the convexity of the loops, and there diverge to enter the 

 hard dentine, through which they then proceed in a parallel 

 course, forming the calcigerous tubes of that substance. These 

 tubes are almost as minute as in the Sloth, having a diameter of j^th 

 of an inch, but have a straighter course ; throughout the greater 

 proportion of the hard dentine, they run at right angles to the plane 

 of the cement. In the teeth of the Mylodon Darwinii, I observed in 

 the hard dentine a few small vascular canals, which had an irregular 

 course, occasionally anastomosing together, and branching at acute or 

 right angles ; their diameter equalled that of five or six of the calcigerous 

 tubes : they were more numerous near the central part of the hard 

 dentine, were filled with dark carbonaceous matter, and, being continued 

 from the larger vascular canals of the soft dentine, established a com- 

 munication between them and the vascular canals of the cement. 

 In the Sloth, no vascular canals have been detected in the hard 

 dentine or in the cement ; in the Mylodon a few of these canals, 

 of larger diameter than those of the hard dentine, are present in 

 the cement(l). They are directed towards the plane of the dentine, 

 and are most conspicuous near that substance ; but do not form 

 a series of loops, like the vascular canals of the central substance. 



(1) Memoir on the Mylodon, pi. xxiv. fig. 3, d. 



