XVIU INTRODUCTION. 



and osseous tissues is gradually effaced, the medullary canals of the 

 vascular dentine, though in some instances straight and parallel and 

 sparingly divided or united, yet are generally more or less bent, 

 frequently and successively branched, and the subdivisions blended 

 together in so many parts of the tooth as to form a rich reticulation. 

 The calcigerous tubes sent off into the interspaces of the net-work 

 partake of the irregular character of the canals from which they 

 spring, and fill the meshes with a moss-like plexus. (1) 



Closely analogous to this modification of the vascular dentine, 

 but differing in the presence of the radiated cells, is the tissue into 

 which the residue of the pulp is converted in the teeth of certain 

 reptiles, as the Iguanodon, Hylseosaurus and Ichthyosaurus, and of 

 those of a few mammalia, as the Cachalot (2). This tissue approaches, 

 in the combined presence of medullary canals and calcigerous cells, 

 as closely to that of the skeleton of the species in which it occurs, 

 as the reticulate modification of the vascular dentine in the teeth 

 of fishes does to the osseous tissue of their skeleton. It has been 

 uniformly described by the authors who have observed it, as Cu- 

 vier(3) and Conybeare,(4) as the result of ossification of the pulp. 



If the first described modification of vascular dentine, which 

 forms the chief part of the teeth of the Sloths and Megatherium, 

 be regarded as a fourth dental tissue, this second modification of 

 vascular dentine, from its closer resemblance to bone might be reck- 

 oned as a fifth ; in proportion, however, as it resembles bone, so 

 likewise it approaches to the structure of cement. 



(1) See Plates 6, 7, 53, 54, 55, 



(2) Plate 89, fig. 2, c. 



(3) Lecons d'Anat. Comp. l^ ed. torn, iii, p. 113 ; Ossem. Fossiles, 2«. ed. torn. v. 2^. partie, 

 p. 274. 



(4) " The teeth in these genera (the Lacertae) become completely solid, its interior ca\dty 

 being filled up by the ossification of the pulpy substance." — Trans. Geol. Soc. vol. vi. p. 106. 



