62 ODONTOGRAPHY. 



and some others of the Pachyderinata, the cement, by its perpendicular 

 interposed layers, constitutes a substantial part of the body of the tooth, 

 as well as a protecting covering to its surface. A great portion of the 

 Mastodon tooth (Plate XX. Fig. 1) is formed by dentine. The mammillary 

 eminences or mastoid projections also have a basis of the same substance ; 

 but they are invested with a covering of enamel, which, in molar teeth in 

 my possession, measures from the sixth to the fourth of an inch in thickness. 

 In teeth which have been worn, the enamel is ground down in various 

 degrees; thus altering the surface of the crown to an appearance approxi- 

 mating, in the Mastodon Giganteus, to the lozenge-shapecl ridges of the 

 African Elephant, 

 wear. The worn surface or "usure" as denominated by the French writers, 



of the cusps of the mammillaB, is frequently employed to express specific 

 characters. The form of these surfaces depends on the form of the emi- 

 nences. If the latter are flattened, as in the mammilla? of Mastodon 

 Giganteus (Plate X.), the worn surfaces will be oblong; if rounded, as some 

 of those in the Baltimore tooth (Plate XXVL), they will be circular ; if 

 indented, the surface will be undulated according to the indentations ; 

 and, where these are numerous, it will be irregular. These irregularities 

 are formed by the worn edge of the enamel, which is in some species, 

 as the Mastodon Andium, fringed. The figure of the worn surface will also 

 be varied by the depth of wear, as shown of the Mastodon Giganteus in 

 the plate already referred to. A slight wear will produce a linear form 

 on the surface ; a greater wear, a diamond-like or lozenge-form figure ; and 

 a very deep wear, an oval, circular, or triangular figure. In the narrow 

 teeth, in consequence of the lateral sinuosities, the worn surface will gene- 

 rally present a trifoliated figure, like a leaf of clover. 



