396 VERTEBRATE ZOOLOGY 



all probability an extraordinarily keen sense of smell plays a prom- 

 inent part in their memory, an enemy being associated with a special 

 odor. Even in human beings, whose sense of smell is at best rudi- 

 mentary, memories of all sorts are inextricably bound up with odors. 



Elephants live to a great age, probably in the neighborhood of two 

 hundred years. In this connection the peculiar arrangement of the 

 molar teeth is of interest; for as the molar teeth that first emerge are 

 worn off by long years of use other molars gradually replace them. 

 The grinding teeth are arranged as though in the arc of a circle, so 

 that only two or at most three on each jaw are in contact at one time. 

 When the front ones wear out the rest move up and take their places, 

 until in very old animals only the last teeth are present. This denti- 

 tion is by far the most specialized found among vertebrates. 



Among the best known recently extinct types of elephants are 

 the mammoth and the mastodon. The mammoth was more nearly 

 like the Indian elephant than any other species, but was much larger. 

 Its tusks were enormous, one being known to weigh two hundred and 

 fifty pounds. These tusks are extremely durable as is demonstrated 

 by the fact that much of the ivory now in use in the form of billiard 

 balls, etc., has been made from them, though their original owners 

 have been dead for thousands of years. The mastodon was about as 

 high as the Indian elephant, seven to nine feet, but was much more 

 stockily built and longer bodied. The tusks were sometimes as much 

 as nine feet or more in length. 



The evolution of the peculiar characters of modern elephants is 

 well shown in a series of extinct forms, as represented by Lull (Fig. 

 203). The earliest proboscidian appears to have been a form like 

 Maeritherium (Fig. 203, F'), which, though rather generalized in most 

 respects, shows the beginnings of elephantine characters in the air 

 cells in the back of the skull, in the enlarged second incisors or in- 

 cipient tusks, and the primitive lophodont molars. It was, however, 

 only about three and a half feet high. Transitional stages are shown 

 in Palceomastodon (Fig. 203, E'), in Trilophodon (Fig. 203, D'), and 

 in Stegodon (Fig. 203, C), in which all of these characters have 

 approached several steps nearer the present condition, as shown in 

 upper figure (Fig. 203, A'). 



Okdeb 12. SiRENiA (DuGONGS AND Manatees). — The sirenians are 

 now looked upon as an aquatic offshoot of an early ungulate stock 

 distantly related to the proboscidians. The traditional position of 



