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these contoured lakes is the dentinal substance of the tooth, sur- 

 rounded again by the outer covering of enamel. The space between 

 the outer and the inner contours is narrowed in each case at a 

 certain point, suggesting a "col"; while it broadens out at other 

 places, suggesting the former sites of cusps or hills. In neighs 

 bouring sections (B, C) we rise above the level of the cols, find a 

 way open to the valleys, and see the separate transverse mountain- 

 ranges (or lophs) of which the tooth is composed. The general plan 



Fig. 444. Upper molar teeth of elephants. A, E. meridionalis (Pliocene), largest 

 of elephants. B, Indian, and C, African elephants. 



of this tooth is characteristic of the Perissodactyles ; but the varying 

 steepness of the hills, the depth of the valleys, and the amount of 

 abrasion or erosion, lead to an infinite variety of patterns, varying 

 with the species, with the age of the animal, and with the order 

 of succession of the particular tooth ; and so rendering it (as Osborn 

 says) "one of the most difficult objects to define and describe in 

 the whole field of vertebrate palaeontology*." 



In Elasmotherium the hillsides are ridged and channelled, and 

 their contours folded or sinuous accordingly. In Rhinoceros broad 

 gaps replace the narrow cols, and certain jutting crags figure on 



* H. F. Osborn, Equidae of the Oligocene, etc., Mem. Amer. Mus. of N.H. (n.s.) 

 n, p. 3, 1918. 



