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 Macrauchenia 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’ 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-lke 
than Horse-lhke. On the other hand, the proportions of femur 
to tibia are more Horse-lhke. 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 TZhoatherium, 
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, Protorotheriwm, we have 
the two lateral toes diminishing just as in Anchithervum. 
1 N. Acta Acad. Caes. Leop. Car. xxvii. 1885, p. 238. 
. > | 
