SUBORDER C 



PROBOSCIDEA 



253 



the transverse valleys between them are filled with cement. In Elephas 

 the number of ridges increases with each new tooth, and finally becomes 

 27. The brachyodont teeth of Moeritheriimi and Stegodon become hypsodont 

 in Elephas. The ridges become plate-like and are joined to one another by 

 means of cement, which in early stages also covers the upper part of the 

 crown. By use the surface of these " elasmodont " teeth becomes flat, and 

 shows broad transverse bands of cement alternating with thin lamellae of 

 enamel and dentine, forming a surface well adapted to the trituration of 

 vegetable food. In Dinofherium each cusp becomes joined with the next so 

 as to form narrow, well-defined ridges, similar to those seen in the tapir. 

 Among Mastodons also there are some " tapiroid " forms. 



The number of molars is three in each jaw. In front of these there were 



Fig. 335. 



Vertical longitudinal section of skull of existing Indian elephant (Elephas indicus Linn.), ce, Brain cavity ; 

 CO, occipital condyle ; Fr, frontal ; i, base of tusk (incisor) ; ?)ii, uio, lirst and second molars ; ME, mesethmoid ; 

 Mx, maxilla ; n, narial passage ; Pa, parietal ; Pvix, premaxilla ; So, supraoccipital. 



originally three premolars, but the number of premolar teeth becomes reduced 

 to two. These teeth are much simpler than the molars and replace the three 

 milk teeth. Premolars are lacking in Elephas, Stegodon and most species of 

 MojStodon. All of the cheek teeth are originally in horizontal rows, and the 

 premolars displace the milk dentition from below. In Mastodon there are 

 never more than three teeth functioning at the same time, and in later forms 

 only two, and finally only one. The dental succession takes place during a 

 long interval of time, the newly formed tooth presses upward and forward 

 in an arc and gradually displaces its predecessor. This succession continues 

 during the whole life of the creature, and the cheek teeth are worn down 

 to inconsiderable remnants. The anterior portion of the teeth first becomes 

 worn down by use. 



The skull (Fig. 335) exhibits the following modifications in tracing 



