642 UNGULATES. 



cement is relatively thicker than in the tusks of the Mammoth 

 or Indian Elephant, and in the section from the worn end of a 

 Mastodon's tusk, of which a magnified view is given in Plate 103, 

 figure 1, the minute terminations of the calcigerous tubes of the 

 ivory are seen to be directly continued into the system of fine 

 parallel tubes of the cement. The minuteness of the tubes of the 

 ivory is illustrated by contrast with those of the tooth of the small 

 Marsupial quadruped figured in the same plate. Some of the forms 

 of the radiated cells which are so thickly distributed through the 

 cement of the Mastodon's tusk are shown at c. The irregular 

 disposition of the calcareous salts of these cells give them the 

 appearance of being subdivided, like the granular cell-nuclei of 

 which they are the remains : but their cavity, though irregular, is 

 single, and has generally been filled by the crystalline matter of 

 the fluids percolating the stratum in which such tusks have be- 

 come fossilized. The general character of the microscopic structure 

 of the ivory of the Mastodon's tusk is the same as that of the 

 Elephant. The peripheral extremities of the dentinal tubes are, 

 in some parts of the tusk, straighter than in the rest of their 

 course ; the straighter extremities were those which were first 

 formed in the calcification of the peripheral part of the pulp, and 

 this first-formed ivory is accordingly, in such parts, more like the 

 ordinary dentine, and is analogous to the thin peripheral cap of 

 such substance in the teeth of the Sloth and of some Fishes. 

 The pulp soon, however, becomes subject to that modification 

 of the calcifying processes by which the more tortuous disposition 

 of the tubuli and the more frequent interposition of opake cellules 

 are produced; modifications which, in establishing the characters 

 of ivory, present a step in the transition from true dentine to 

 osteo-dentine. 



By the minuteness and close arrangement of the tubes, and 

 especially by their strongly undulating secondary curves, a tougher 

 and more elastic tissue is produced than results from their dis- 

 position in ordinary dentine ; and the modification which distinguishes 

 * ivory' is doubtless essential to the due degree of coherence of so 

 large a mass as the Elephant's tusk, projecting so far from the 



