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PRINCIPLES OF ANIMAL BIOLOGY 



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. 218,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 de- 

 velopment of cancellate bone in the skull, to which the supporting mus- 

 cles 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 develop- 

 ment 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 



Fig, 219. — Mastodon tooth, showing the enormous cusps on the upper surface. (From a 

 California specimen in the Museum of Geology, University of Michigan.) 



of the Indian elephant. The tusks (C) were much elongated (9 feet 

 or more), but the lower jaw was greatly shortened and the lower incisor 

 teeth were reduced or wanting. The molar teeth (Fig. 218, C, and Fig. 

 219) 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 some of the 

 fossil mastodons. 



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

 teeth bore five or six well-defined transverse ridges (Fig. 218,5). These 

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

 and enclosing a substance known as dentine. Over the enamel in an 

 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 

 mammoths (see Fig. 220 for the tooth of a mammoth). On the whole, 

 Stegodon was intermediate between the mastodons and elephants. 



