I50 EVOLUTION, GENETICS, AND EUGENICS 



which is believed to indicate the existence of a short probosci^i 

 extending at least to the tips of the tusks. 



Trilophodon. — Trilophodon, a great migrant and consequently 

 wide-spread over several continents as stated above, exhibited in 

 several respects a striking advance over Palaeomastodon; but this 

 advance was in the main in the same direction as was indicated by 

 the change from Moeritherium to Palaeomastodon. Trilophodon was a 

 huge animal, nearly as large as modem Indian elephants. The tusks 

 were considerably longer (Fig.34, D') and still bore a band of enamel. 

 The molar teeth were large and greatly reduced in number, so 

 that only two were present at any one time on each side of each 

 jaw. The surface of these teeth bore a somewhat larger number of 

 transverse crests (Fig. 34, D) than were present in the earlier forms. 

 The lower jaw was enormously elongated, so that it projected as far 

 forward as the tusks. The great weight of the lower jaw and tusks 

 was associated with a considerable development of cancellate bone 

 in the skull, to which the supporting muscles of the neck were 

 attached. Presumably there was a proboscis which extended to or 

 beyond the tips of the tusks and lower jaw. 



Mastodon. — The mastodons on the whole represent a line of 

 development which became extinct; but in their incipient stages they 

 appear to have given rise to the succeeding forms leading to the 

 elephants. The body was somewhat larger than that of Trilophodon, 

 being about the size of the Indian elephant. The tusks (C) were 

 much elongated (9 feet or more), but the lower jaw was greatly short- 

 ened and the lower incisor teeth were reduced or wanting. The molar 

 teeth (Fig. 34 J C) were scarcely more complex than earlier forms, and 

 numbered two on each side of each jaw. They were still crushing 

 teeth, and the food must have been tender twigs and succulent plants; 

 indeed, remains of such objects have been found in the region of the 

 stomach of the fossil mastodons. 



Stegodon. — This animal is of interest chiefly because the molar 

 teeth bore five or six well-defined transverse ridges (Fig.34,5). These 

 ridges were due to plates of enamel extending up through the tooth, 

 and inclosing a substance known as dentine. Over the enamel in ah 

 unworn tooth was a thin coat of a third substance called cement, but 

 there was not much of this substance between the ridges. In the 

 latter respect Stegodon differed, as is pointed out below, from the 

 elephants and manunoths. On the whole, Stegodon was intermediate 

 between the mastodons and elephants. 



