1865.] LANKESIER CEAG MAMMALIA. 229 



tig. 5). The area of the pulp-cavity, which is very large, is 

 throughout the whole length filled up, a very small and flat space, 

 which is the only true pulp-ccrviiy, existing at the base. The sub- 

 stance which occupies its place, and which is known in other teeth 

 as " osseo-dentine," appears to be composed of a number of small 

 globular bodies, closely agglomerated and compacted, and presenting 

 that peculiar appearance and structure which is characteristic of the 

 tusk of the Walrus, and was compared by Cuvier, when writing of 

 that animal, to " pudding-stone." Thus : — " L'i voire des defenses 

 du Morse est compacte, susceptible d'un poli presque aussi beau que 

 celui de I'Hippopotame, mais sans stries; la partie moyenne de la 

 dent est formee de petits grains ronds places pele-mele, comme le 

 cailloux dans la pierre appellee poudingue; c'est ce qui le carac- 

 terise. Les dents molaires de- cet animal ont leur axe compose des 

 memes petits grains que celui des defenses. Elles n'ont aucune 

 cavite dans leur interieur " *. 



This structure, which is in reality formed by numerous distinct 

 calcifications around various vascular canals, from which radiate 

 tubules similar to those of the dentine, occupies a large part of the 

 fossil, as it does of the Walrus-tusk, diminishing as the point is 

 approached. The microscopical appearance of this part of the tusk, 

 and of the dentine and cement exterior to it, is shown in the accom- 

 panying drawings of sections, obtained with some difflcultj, from the 

 fossils and from the tusk of the living Morse (PI. X. figs, 4 & 6). 



The dentine which surrounds the peculiar " osseo-dentine," and 

 forms the bulk of the tooth, is very hard and compact, and has a 

 radiated fibrous appearance, owing to the direction taken by the 

 dentinal tubules, which, although excessively minute, are thus far 

 rendered apparent by the selective infiltration of mineral matters. 

 The tubules do not appear to exert the least infiuence on the direc- 

 tion of the fracture of the tooth, which is, as before stated, in 

 longitudinal annular laminae. Deorganized tusks of the Walrus 

 present this same form of disintegration. 



In its microscopical structure, the dentine of the fossil tusks pre- 

 sents a complete resemblance to that of the tusk of the Walrus, 

 which will perhaps be best understood by reference to Plate X. 

 figs. 4 & 6. The dentinal tubes are very nearly of the same size, 

 and equally closely packed, and are connected with stellate lacunae 

 in some numbers near the periphery of the tooth. This structure, 

 which is not peculiar to the Walrus, is nevertheless a test of affi- 

 nity, inasmuch as the form of the lacunae varies in different animals. 

 They ai-e not met with in the tusks of the Proboscidea or the Hippo- 

 potamus, but occur in the curious incisors of the Dugong. The 

 " dentinal cells" of the Crag tusks also resemble those of the Walrus. 



The " cement," as seen in a transverse section of one of the 

 fossil tusks, which was cut at a distance of nine inches from its 

 terminal point, and the diameter of which was there 2 x ^\ inches, 

 appeared not to be more than the sixth of an inch in thickness, 

 whilst the thickness of the " dentine," compared to that of the 

 * Olivier, Le9onB d'Anat. Comparee, torn. iii. (1805), p. 106. 



