172 EVOLUTION AND GENETICS 



primitive. The molars were transversely ridged, but the ridges 

 were made up of associated cusps (Fig. lOOC). They were so small 

 that several were probably in use at the same time. Tusks were 

 sometimes present, although poorly developed, in the lower jaw. 

 Writing of the American Mastodon, Scott says that there is evi- 

 dence to show that "it had a covering of long, coarse hair, and that 

 it fed upon the leaves, shoots and small branches of trees, espe- 

 cially of conifers." The genus became extinct in the Old World 

 before the end of the Pliocene, but migrants which entered America 



by way of the land bridge 

 between Asia and Alaska 

 persisted until the middle 

 of the Pleistocene, and 

 were probably contem- 

 poraneous with the early 

 human inha])itants of the 

 Fig. 98.— Head of Tefralophodon lulli. The continent. Remains in- 

 lower jaw, the longest recorded in any pro- dicate that the heavy 

 boscidean, measured at least six feet in . , „, . , 



length. (From Lull, after Barbour and Kunz.) annuals were often mu'cd 



in sloughs and marshes. 

 An unusually fine skeleton was taken from such a situation in 

 the summer of 1926 at Johnstown, Ohio. The bones were very 

 near the surface and in a well settled region, conditions which 

 emphasize one reason for incompleteness of our knowledge of 

 extinct species, viz., the element of chance in the discovery of 

 fossils. 



From Mastodon to the Elephants. Mastodon produced another 

 genus, Stegodon, which appears in Asiatic deposits of the early 

 Pliocene. Stegodon was much like the modern elephants. Its 

 molars bore more and finer ridges than those of the true mastodons, 

 and the derivation of these ridges from rows of conical eminences 

 is less evident (Fig. lOOB). The worn surface shows dentine 

 plates surrounded by enamel, but the cement is not abundant as 

 in the true elephants. The lower jaw is short and without tusks, 

 and other characters are so close to those of the elephants that 

 Stegodon has been looked upon as congeneric with them. Fossil 

 remains of Stegodon have been found only in southern and south- 

 eastern Asia, which may therefore have been the region in which 

 the true elephants developed. 



From Stegodon it is only a step to the genus Elephas, including 



