210 INTRODUCTION TO EVOLUTION 



gether by cement. Each plate or lamella has a core of dentine surrounded 

 by enamel. The composite structure produced by cementing together such 

 lamellae presents a broad grinding surface on which transverse ridges of 

 enamel rise above the general surface, owing to the more rapid wearing 

 away of the softer dentine and cement (Fig. 10.10). 



As everyone knows, most mammals have, during the course of their 

 lifetimes, two sets of teeth: the "milk teeth" or deciduous teeth and the per- 

 manent teeth. The permanent teeth replace the milk teeth vertically; a 

 permanent premolar in the upper jaw, for example, develops above the 

 corresponding premolar of the milk set and eventually replaces that tooth 

 by moving down into the position formerly occupied by it. Thus, early in 

 life a typical mammal has a complete set of milk teeth all in use at one 

 time, later in life a complete set of permanent teeth all in use at one 

 time. Arrangements are quite otherwise in elephants. While some of the 

 molars are identified as milk teeth, others as permanent teeth, the 

 individual teeth succeed each other in series, one at a time, rather than 

 as complete sets. Thus typically at any given time only four molars are in 

 use, one in the upper jaw and one in the lower jaw on each side. As the 

 molars wear out they are replaced by others, but replacement is 

 longitudinal, not vertical; the new molar is pushed forward from the rear 

 of the jaw. Fig. 10.9 shows this method of replacement in the upper jaw. In 

 the figure the third molar (m^) is present as a worn-out remnant; the 

 fourth molar is shown as the functional one, and the fifth molar is shown 

 forming above and behind the fourth one, as a reserve to replace the latter 

 when it in turn wears out. 



Moeritherium 



Having reviewed some of the distinctive characteristics of elephants 

 we turn our attention to the earliest known ancestral proboscidean, 

 Moeritherium. As in the evolution of the horse, our starting point is a 

 relatively small animal showing only the beginnings of the specializations 

 of its descendants and living early in the Cenozoic era. Moeritherium 

 lived in Africa in late Eocene and early Oligocene times. It was about the 

 size of a modern tapir, perhaps 3 feet in height, and probably had 

 a tapirlike short proboscis (Fig. 10.11), as indicated by slight recession of 

 the nasal openings of the skull. The second upper incisors were beginning 

 to form tusks (Fig. 10.12). Short tusks on the lower jaw will also be noted; 

 they projected forward in a somewhat spoutlike fashion. They are unlike 



