^n AMi.nn A\ m lsfam (.rim: li:m lets 



Tetralophodon, Dibelodon, etc. Tlic miincroiis icinains of inastodons in 

 I lie Mioccjic jmkI PliociMic of Kiirojx', Asia and \ortli America sliow a |)r()- 

 ^rt'ssi\(Mli\(M'^('n((' iido scNcral distinct types. In one scries tlie elonpition 

 of the jaw. already ^reat in PahronKtstodon, is carried loan extreme, culmin- 

 ating in >^u(•ll tyj)es as Tctrdlop/iodon nun pester and longirostris. The inter- 

 mediate mohirs haxc four crests, the hist mohn* n|) to ei^lit, l)ut the cresting 

 remains imj)erfect and the cross vaUeys are l)h)cke(L This is the Tefralopho- 

 r/o// t»ron|) of the hite Miocene and early IMiocene of Europe, Asia and North 

 Ainci-ica. In a second urouj), cliiefly found in the New World, the u|)|)er 

 tusks are enlarged and often cur\'e uj)ward; tlie l)and of enamel on their 

 outer surface is sometimes retained l)ut more often disjippears com|)letely. 

 The lower tusks are lost, the jaw Nhortencd, the j)osterior <>rindin<i- 

 teeth j)r(),uressi\-ely i'nlar<;ed, while tl:e anterior ones are early lost. 

 The teeth remain short-crowned although a small amount of cement 

 is often prescMit in the \ailevs. This is Dibelodon, common in the 

 North American Pliocene and South American Pleistocene. All the 

 Mastodons of South America l^elong in this group. In a third 

 group Ixhi/ncoflu'riiim the front of the lower jaw is IxMit downward at a 

 sharp angle, the lower tusks large and straight. Little is known of this 

 groui), found only in ^Mexico and Texas. 



Mastodon. A fourth grouj) leads up into the great American INlas- 

 todon. In it the jaw is shortened, the lower tusks become vestigial, the 

 cross crests of the molar teeth are perfected into a chopping tapir-like 

 tyj)e. The uj)per tusks are large, curve u])ward with a spiral twist as in 

 the ele})hants and have no enamel. Early stages are seen in Trilophodon 

 fiirlceti.sis of Europe, T. hrevidens of Montana and T. serridens of Texas, of 

 Miocene age; M. borsoni of the Pliocene and Pleistocene of Europe and 3/. 

 (uuerieanus of the northern Pleistocene represent its culmination. 



Stegodon, Elephas. The fifth and most imi)ortant group leads up 

 through the xarious species of Stegodon of the Pliocene of India into the 

 elephants. In this group the jaw is greatly shortened, the lower tusks com- 

 ])l(^t(Oy (lisai)pear and the u])])er tusks as in the American Mastodon are 

 enlarged and curve u])war(l. The ])osterior grinding teeth are progressively 

 eidarged while theanterioronesareearly lost. The crests of the molar crowns 

 hecome progressively higher, narrower and more munerous. while the valleys 

 are filled with cement, so that during wear the grinding surface presents cro.ss 

 ridges of enamel alternating with softer dentine and cement. The skull is 

 greatly shortened, the trunk lengthened, the cranium built out w ith cellular 

 bone, reaching finally the extreme s])ecialization seen in the Pl(Mstocene 

 and modern elephants. The length of the limbs is also nnich increased, 

 coordinafcK" with the lenuth of trunk. 



