146 EIGHTH REPORT — 1838. 



of the coarser canals. The fine crust of ivory above mentioned is 

 formed by minute tubes directly continued from the finer ramifications 

 of the large canals of the central substance, and terminated in plexus of 

 still finer tubes, which at length escape the highest magnifying powers. 

 The fang, or inserted part of the tooth, of the sloth is coated with a 

 ayer of cnista petrosa, which is characterized by large canals and abun- 

 dant Purhingian corpuscles. There is no enamel in the composition of 

 these teeth or of those of any of the existing Edentata. 



Megatherhim. — Microscopic examinations of the structure of the 

 tooth of this extinct mammifer have undeceived me with respect to its 

 conformation ; the thin dense layer between the crusta petrosa and- the 

 internal substance composing the body of the tooth is not enamel, but 

 a layer of ivory composed, like the dense ivory of the teeth of other 

 Mammalia, of minute tubes having a parallel course at right angles to 

 the surface, and minutely undulating in that course, and corresponding 

 with the thin cylinder of true ivory in the tooth of the sloth. The cen- 

 tral part of the body of the tooth consists of a coarser ivory, much 

 resembling the teeth oi Psammodus or Myliobatis, among fishes. It is 

 traversed by large medullary canals parallel to each other and to the 

 finer ivory tubes, having angular interspaces equal to one and a half 

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

 loop whose convexity is close to the origin of the fine ivory tubes, as if 

 each pair so joined was composed of one reflected canal. Some, how- 

 ever, are continued across the fine ivory, and anastomose with the corre- 

 sponding canals of the ccementum ; the interspaces of the coarse ivory 

 tubes appear at first view granular, but thej' are principally occupied 

 by reticular branches given off from the canals : some of these anasto- 

 mosing branches are seen coming off" from the concavity of the loops, 

 and retrograding. Numerous minute cells are scattered about the ter- 

 minal loops of the medullary canals of the coarser ivory. The origin 

 of the fine ivory tubes is from the convexity of the peripheral loops of 

 the above medullarj' canals. The ivory tubes are separated by inter- 

 spaces equal to one and a half their own diameter ; they divide and 

 subdivide, growing smaller and more Avavy toAvards the periphery or 

 camentum ; here their terminal branches assume a bent direction, and 

 form anastomoses, dilate into small cells, and many are clearly seen to 

 become continuous with the radiating fibres or tubes of the corpuscles 

 of the contiguous camentum. The cement is traversed by large canals 

 running, like the canals of the coarse ivoiy, parallel to each other and 

 to the course of the fine ivoiy tubes, with interspaces of about five times 

 their own diameter, occasionally, but rarely, dividing dichotomously, — 

 in which case the branches usually anastomose and form loops with the 

 convexities towards and close to the outer layer of fine calcigerous 

 cells, in which the fine ivory tubes terminate. The cement difi'ers 

 from the coarse ivory in the fewer number of canals, and more espe- 

 cially by the presence of the bone corpuscles or radiated cells in the 

 interspaces of the canals. The irregular tortuous fine tubes forming a 

 network in the interspaces, and especially those proceeding from the 

 convexities of the loops, are much more distinct than the correspond- 



