MAMMOTHS AM) MAST()/)()\S 21 



A<i:aiiist this view it lias hocii pointed out thai none of these special- 

 izations are earried in Mwritherium to a dej^ree thai would ser\'e to place 

 it eonelusively or definitely as proboscidean. All of thciu may readily he 

 matched in various other living or extinct niamnials which certainly have 

 no proboscidean affinities. M(rriiherium, although suj)])()sed to be the 

 immediate ancestor of Pala'omastodon, differs from it more widely than it 

 does from the Miocene mastodons, separated by a much wider gaj) in time. 

 And some of the differences ajjpear to be not primitive characters but 

 divergent speeializations from the primitive ungulate tyj)e. 



VI. THE EVOLUTION OF THE PROBOSCIDEA. 



IX the })receding sections we have sketched briefly what is known 

 about fossil elephants and mastodons, and their ancestors in the 

 Tertiary period. The principal types are shown in the American 

 Museum exhibit. 



These fossil skulls and skeletons carry the ancestry of the proboscideans 

 as far back in geological history as the Eocene. Although we do not regard 

 Mopriiherium as a direct ancestor of Paloeomasiodon, it represents in many 

 respects the primitive type of ungulate from which the proboscideans were 

 derived. 



Moeritherium is an animal of quite moderate size; it has no indications 

 of a trunk; the head is long and low, the brain-case small with little or 

 nothing of the cellular bony cover that builds up the later proboscidean 

 skull into so remarkable a bulk. The teeth have departed relatively little 

 from the primitive type common to all early mammals, of three incisors, 

 a canine, four premolars and three molars in each jaw^ — 44 teeth altogether. 

 A pair of upper and low^er incisors have been enlarged, the third lower 

 incisor lost, and each molar has two pairs of cusps on the crown, not yet 

 fully united into cross crests. The posterior premolars have taken on in part 

 the character of the molars. Little is know^n of the skeleton save that the 

 limbs were of moderate length, the knee much more bent than in the later 

 proboscideans. 



Palaeomastodon is clearly of proboscidean type, and there is a wide 

 structural gap between it and Moeritherium. The enlarged pair of incisors 

 are much elongated and the enamel confined to the outer face. The long 

 forward reach and moderate downward curvature of the upper incisors, 

 the great length of the front of the jaw and close set lower incisors, pro- 



2 0sbom, 1909, Nature, Vol. Ixxxi, p. 139. The problem of the affinities of Moeritherum 

 is here discussed at some length. 



Professor Osbom concludes that: "It would not be far from the truth to say, from our present 

 knowledge of the animal, that Moeritherium is an offshoot of the Proboscideo-Sirenian stock, 

 with slightly nearer kinship to the elephants than to the Sirenians." See also Dr. Andrews' reply 

 to the above (ibid., p. 305), concluding as follows: "On the whole it seems that the weight of 

 evidence is in favor of regarding Moeritherium as a proboscidean, though perhaps not on the 

 direct line of ancestry of Palaeomastodon, and retaining some characters of the original Probosci- 

 deo-Sirenian stock." 



