IGUANIANS. 245 



comminution of the food by these instruments ; and their summits 

 are rather chipped off than ground down by use. The appearance of 

 abrasion is greatest in the posterior teeth, especially in the Iguana 

 cornuta, in which the crowns of the teeth are thicker than in the 

 Iguana tuherculata, and make a nearer approach to the very remark- 

 able form of tooth that characterizes the gigantic Iguanodon. 



Before, however, proceeding to describe the teeth of this extinct 

 lizard, I shall offer a few observations on the microscopic structure of 

 the teeth of the existing Iguana. In both the common and horned 

 species, the teeth consist of a body of simple compact dentine, with the 

 crown covered externally by a thin layer of enamel, and the fang with 

 an investment of cement. The dentine, viewed by transmitted light in 

 a thin horizontal section, exhibits minute calcigerous tubes in a clear 

 substance, radiating from a simple conical pulp-cavity, w^hich is 

 widely open at the base of the tooth and continues in a linear form 

 into the crown of the tooth : the calcigerous tubes at the base of the 

 tooth proceed in an irregular sinuous course at right angles to the 

 axis of the tooth : above this part they sweep outwards in a graceful 

 curve, with the concavity turned towards the base of the tooth : as 

 they approach the summit of the tooth they gradually incUne towards 

 it, and those from the apex of the pulp-fissure proceed directly in the 

 axis of the tooth : throughout their course the calcigerous tubes are 

 disposed in minute undulations, and they send off from the concave 

 side of the primary flexures numerous short parallel branches at an 

 angle of 45° : these branches rise less regularly the nearer the main 

 tube is to its origin from the pulp-cavity. The diameter of the calci- 

 gerous tubes is siith of an inch : their interspaces are equal to be- 

 tween three or four of their diameters. 



In general they do not divide until within a short distance from 

 the periphery of the tooth, near which they subdivide frequently ; 

 the terminal branches of the different layers decussate each other. 

 The tubes at the base of the tooth divide nearer their origin and more 

 frequently ; which, with the large oblique branches, and the stronger 

 undulations of the main tubes occasions the interwoven appearance 

 represented at PI. 65 b., fig. 3. 



The pulp-cavity in old teeth becomes occupied by a coarse bone, 



