688 The American Naturalist. [August, 
ginglymoid articulation: of the astragalus with the navicular in 
one, the anteroposterior, plane. In the Condylarthra this articu- 
lation is ball and socket, or nearly universal, as in the Unguicu- 
lates generally. The articulation in the Litopterna is of ungu- 
late character, and has the same functional value in the fully 
developed forms, such as Epitherium, as in the Artiodactyla. In 
the known forms the fibula articulates with the calcaneum, 
another point in which they differ from the Condylarthra. A 
peculiar character, said by Ameghino to characterize this sub- 
order, is the presence of four roots of some of the inferior pre- 
molars and molars. This authority also states that they do not 
possess a clavicle. 
The three families differ as follows. 
Superior molars essentially tritubercular, the two 
external cusps modified into Vs; inferior mo- 
lars with Vs; Proterotheriude. 
Superior molars with straight external wall and 
cross-crests, no Vs; Astrapotherude. 
Superior molars with subequal external Vs and ; 
cross-crests ; inferior molars with Vs; Macraucheniide. 
The dentition of the Proterotheriidæ could be easily derived 
from that of the Periptychidæ of the Condylarthra by the modes 
of complication usual in other Ungulata. On the other hand 
_ that of the Macraucheniidæ could have been derived from that of 
the Meniscotheriide. The dental type of the Astrapotheriide 
could have been derived from that of Protogonia by a.process 
similar to that by which the Rhinocerotine line arose from a 
primitive quadritubercular diplarthrous form. The parallel pre- 
sented by the succession of the Litopterna in time as compared 
with that of Diplarthra is most remarkable. The resemblances 
in the dentition are such that the early students of the paleon- 
tology of Argentina referred members of the Proterotheriidze and 
Macraucheniidz to the Perissodactyla and Artiodactyla, and later 
authors have referred one of the Astrapotheriidz to the Rhinocero- 
tid. In the last case the resemblance may be traced in all parts 
of the molar teeth, even including the plication of’the anterior part 
of the external wall of the crown. The entire suborder is a 
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