TRANSACTIONS OF THE SECTIONS. 14^ 



ing tubes in the coarse ivory. The primary branches of the canals go 

 off generally at right angles. In a few places I have distinctly seen 

 the large canals of the ccemtntum traversing the subs.tance of the fine 

 ivory to anastomose with tliose of the centi'al coarse ivory. 



We have thus, then, in the tooth of the Megatherium, an unequivo- 

 cal example of a course of nourishment of the teeth distinct from and 

 superadded to that which proceeds from the surface of the pulp and 

 the cavity of the fang in which it is lodged, viz. by a direct communi- 

 cation between the vascular canals of the external organized ccEmentum 

 and the tubuli of the ivory. Retzius observes of the human tooth, that 

 " the fine tubes of the ctsmentum enter into immediate communications 

 with the cells and tubes of the ivory, so that this part can obtain from 

 without the requisite humours after the central pulp has almost ceased 

 to exist." In the Megatherium, however, those anastomoses have not 

 to perform a vicarious office, since the pulp maintains its full size and 

 functional activity during the whole period of the animal's existence. 

 It relates to the higher organized condition, and doubtless to the higher 

 vitality of the entire grinder in that extinct species. 



The views entertained by Cuvier of the affinity of Megathernmi to 

 Bradypus, derive full confirmation from the microscopic investigation 

 of its teeth. It needs but to compare the preceding description with 

 that published by Retzius of the structure of the teeth of the Armadillo, 

 to perceive how much more closely the Megathere resembles the Sloth 

 in the structure of its teeth. The Megatherium has ten teeth in the 

 upper jaw, five on each side ; differing slightly in form and size, but all 

 presenting the same characteristic vascular structure as above described. 

 The structure of the coarse central ivory may be compared with that of 

 which the entire tooth of the Orycteropus is composed, with these dif- 

 ferences, — first, that the parallel medullary canals and their systems of 

 calcigerous tubes are not separated from their neighbours by a layer of 

 cccmentum, and, secondly, that the medullary canals anastomose at their 

 peripheral extremities. 



Toxodon. — The teeth of this extinct animal have an external but in- 

 complete investment of enamel, which is deficient for a small extent at 

 the anterior and posterior sui'faces of the tooth ; but these parts, as 

 M'ell as the enamel, are covered with a thin exterior layer of ccementum. 

 The body of the tooth is composed throughout of compact ivory, con- 

 sisting of minute wavy calcigerous tubes, j^th of a line in diameter at 

 their origin, which radiate in directions vertical to the superficies of the 

 tooth, or of the inflected fold of enamel, from the central pulp-cavity. 



In the discontinuity of the enamel surrounding the ivory of the tooth, 

 the Toxodon differs from all known Pachyderms, and exhibits an ap- 

 proach to the Rodentia and Edentata. 



In the Leopard, the tubuli of the canine teeth are chiefly remarkable 

 for the number of their ramifications, and the beautiful curvatures of 

 the same. In the Mole, the main tubes are remarkable for their width 

 and shortness; they are as large at their commencement as in the 

 human tooth, but soon divide at their extremities into a number of 

 smaller branches, which again subdivide, the terminal twigs anasto- 



L 2 



