268 ONE-TOED LITOPTERNA CHAP, x 



fossa, that it cannot be regarded as anything more than a mark 

 of specialisation. It is, in fact, the case that the Macraucheniidae 

 are in many points specialised, while retaining many primitive 

 features of structure. 



The chief primitive features are : the non-alternating positions 

 of the wrist- and ankle-bones ; these, of course, interlock in the 

 Perissodactyles of to-day and in many extinct families. Then 

 the absence of a diastema in the tooth series, coupled with the 

 presence in Mavrauchenia of a complete dentition. The small 

 brain may be referred to the same category. Macrauchenia 

 must have been a strange -looking animal. It walked upon 

 three toes on each limb; the skull was Horse -like in general 

 form, but the nostrils are removed to a point about as far back 

 as in the Whales or nearly so, the -nasal bones being correspond- 

 ingly reduced. This it is thought argues a proboscis. The 

 humerus is particularly compared by Burmeister 1 to that of a 

 Horse. The radius and ulna though both well developed are 

 fused. The neck is long, and, as in the Camel, the vertebral 

 arteries run inside the neural arches. Since the fore-legs seem 

 to have been rather longer than the hind-legs, though only very 

 slightly, and the neck was long, the animal may have presented 

 some likeness to the Giraffe. It is interesting to note that in the 

 proportions of humerus to ulna this animal is more Lama -like 

 than Horse-like. On the other hand, the proportions of femur 

 to tibia are more Horse-like. The remains of the creature are 

 limited to South America, and to quite superficial deposits. It 

 is evidently a specialised type, and has pursued a course parallel 

 to that of the Horse. Much nearer to the Horse however, but 

 apparently by convergence only, is the genus Thoatherium, 

 usually placed in a separate family, the Protorotheriidae. In this 

 creature, which has many archaic characters, the toes are reduced 

 to one in each foot. In an allied form, Protorotherium, we have 

 the two lateral toes diminishing just as in Anchitherium. 



1 N. Ada Acad. Cacs. Leap. Car. xxvii. 1885, p. 238. 



