76 SAUROIDS. 



like coating of the tooth receives numerous similar parallel ramuscules 

 from the stratum of calcigerous cells which forms the boundary 

 between this external layer and the ivory of the tooth. The pulp- 

 cavity, which has a compressed ovate section, is gradually contracted 

 and finally obliterated in the basis of the tooth, where it divides into 

 numerous minute canals which subdivide and anastomose as they 

 penetrate deeper into the jaw, and finally terminate in minute tor- 

 tuous canals, which become continuous with those of the coarse 

 osseous substance in which they are imbedded. All the preceding 

 branches of the pulp-cavity are the centres of radiation of as many 

 systems of calcigerous tubes, similar in size to those of the body 

 of the tooth. The primary curvatures of these tubes are shorter, and 

 the interspaces somewhat wider ; the calcigerous cells in which the 

 fine branches of these calcigerous tubes terminate, are also coarser. 

 The interspaces of the divisions of the base of the tooth are occupied 

 with the coarse cellular bone of which the jaw is composed. 



The teeth of the Rhizodus thus include two very different types 

 of structure, which pass insensibly into one another ; the exserted 

 body of the tooth is like the dense ivory of the ordinary mammiferous 

 or saurian tooth, the inserted basis presents the compound structure 

 of the teeth of the Myliobates and Orycteropus. Each division of the 

 tooth's basis, however, presents in the Rhizodus greater strength and 

 density than the corresponding parts of the tooth of the Myliobates, 

 in consequence of the greater minuteness, increased number, and 

 more aggregated and parallel disposition of the calcigerous tubes, and 

 the smaller diameter of the medullary cavity in the former species. 

 The advantage which was obtained for the teeth of the extinct 

 carnivorous Sauroid by the root -like implantation above described, 

 in resisting displacement and fracture, is too obvious to need further 

 illustration. Finally, as the teeth of no species of reptile or fish are 

 similarly subdivided and implanted in the jaws, the teeth of the 

 Rhizodus, notwithstanding the organization of the crown, may be 

 readily and certainly distinguished from them. 



The large conical laniary teeth of the extinct genera Holoptychus 

 and Megalichthys present a similar structure to that of the tooth 

 above described. The whole body of the tooth is composed in 



