194 



KNOWLEDGE. 



[August 1, 1890. 



ones the number of ridges is generally either three or four 

 (Fig. 13), with additional imperfect ridges at the two 

 extremities. In such a tooth (Fig. 18) it will be seen 

 that the transverse ridges are low and roof-like ; the 

 valleys separating one ridge ft-om another being broad and 

 comparatively shallow, without any of the substance 

 known as cement at their bases. When tlie enamel on 

 the summit of such ridges is worn through by the abrasion 

 of the teeth of the upper jaw against those of the lower, 

 oval or ti-efoil-shaped islands of the imderlying dentine 

 are revealed, as in Fig. 14. The latter figure exhibits 

 the last tooth of a Mastodon in which there are five com- 

 plete transverse ridges, this being the most complex form 

 of tooth found in any of the Mastodons. 



In certain Tertiary rocks in India lying along the 



southern flanks of the great range of the Himalaya there 

 are, however, found enormous numbers of teeth of peculiar 

 species of Elephants, which, whOe resembling in many 

 respects those- of the Mastodons, have a considerably 

 greater number of ridges. Moreover, the ridges them- 

 selves are relatively narrower and taller, so that the depth 

 of the intervening valleys is likewise in- 

 creased. These valleys also contain a certain 

 amount of the third constituent of mammalian 

 teeth — the cement — so that their bases are 

 partially filled up by it. 



From the teeth of these Intermediate Ele- 

 phants, as they may be conveniently called, 

 the step is very short to those of the modern 

 or true Elephants. In these, as shown in 

 Fig. 15, the difference from the teeth of the 

 Mastodon is so great that without the inter- 

 mediate forms it is difficult to trace the 

 correspondence between their respective ele- 

 ments. In the true Elephants, indeed, the 

 transverse ridges of the teeth have not only 

 been greatly increased in number, so that 

 there may be as many as twenty-five in 

 the last tooth of certain species, but they 

 have also been so much increased in height and narrowed 

 in width that they assume the form of thin plates, which 

 may be six or eight inches in height, and of which the 

 sides are almost parallel. The valleys between these 

 plates, as they may now be called, have likewise become 

 thin and deep slits, which are completely filled to their 

 very summits with the cement. Thus, comparing Fig. 15 

 with Fig. 14, it will lie apparent that each of the 

 elongated discs seen in the former, which consist of a 

 layer of enamel surrounding a strip of dentine, correspond 



to the transverse ridges of the latter ; while the space 

 between the discs in Fig. 15, which are composed of 

 cement, represent the open valleys of Fig. 14. 



The surface of such an Elephant's tooth forms, indeed, 

 a millstone most perfectly adapted for grinding vegetable 

 substances, consisting as it does of parallel ridges com- 

 posed of elements of different degrees of hardness. Such 

 a tooth, with its height of nearly eight inches in some 

 sj)ecies, takes many years to wear away ; and with a suc- 

 cession of six of these teeth gradually increasing in size 

 and complexity from the first to the last, we are well able 

 to understand how the Indian Elephant can live fully to 

 the age of a century. It is also equally evident that the 

 more simple and lower-crowned teeth of the Tertiary Mas- 

 todons must have been worn away at a far more rap'd 

 rate ; so that we are justified m saying that 

 these animals could not have attained any- 

 thing like the length of life enjoyed by their 

 modern descendants. 



There is a considerable amount of variation 

 in the structure of the grinding teeth of 

 the trite Elephants, although all of them 

 resemble to a greater or less degree the speci- 

 men represented in Fig. 15. In the African 

 elephant, however, the discs of dentine, sur- 

 rounded by their border of enamel, are much 

 wider in the middle than in the figured 

 tooth, and thus assume a lozenge shape. 

 In this respect, therefore, the African elephant 

 is a more generalised or old-fashioned kind 

 of animal than his Indian cousin ; and we may 

 observe, in passing, that the African continent 

 is now remarkable for containing a number 

 of old types, such as Hippopotamuses, Giraffes, 

 and Aard-Varks, which ha\e totally disappeared 

 from other regions, although, as we know from their fossil 

 remains, they were once widely spread over the globe. 

 Thus, Hippopotamuses (or shall we say Hippopotami ?) once 

 ranged over the greater part of Europe, extending as far 

 north as the southern parts of our own islands, and were 

 also common in northern India ; the same being true of 



Unwiirn Condition. 



■fiiE La''! Li-n T^iifR T )oTn if 4 "\I isToDON, with the emimel of 

 the two first ridges perforated by wear. 



the Giraffes, with the exception that their remains have 

 not hitherto been found in Britain. 



The most complex type of teeth is, however, attained 

 by the Indian Elephant, and the closely allied Mammoth, 

 which in the latest geological epoch ranged over the 

 greater part of Europe, and whose frozen carcases are 

 from time to time washed out from the so-called " tundras," 

 or superficial de^josits of Siberia, to be exposed to human 

 view after having been buried for countless centuries. In 

 these two species the plates of the teeth are narrower and 



