58 
TERTIAET VERTEBRATA OE THE EAYtM. 
'I'liis bone differs widely from the navicular of Eleplias in articulating with the 
cuboid by its outer face only, and not being interposed between that bone and the 
astragalus. From the navicular of the Amblypoda it differs mainly in the depth 
and ])rominence of the ectocuneiform region, which is such that the outer face of 
the navicular articulates with the cuboid by both its upper and lower edge. In 
Uintatherium and Corypliodon the navicular articulates with the upper edge of the 
cuboid only and the lower portion of the inner side of that bone joins the 
ectocuneiform. It may be suggested that it is just possible that in the bone here 
described and figured the ectocuneiform may be fused with the navicular, but there 
is no actual trace of such a fusion having taken place in any specimen examined. 
Text-fig. 34. 
B. X I 
Right cuboid of Arslnoitherium zitteli : A, navicular face ; B, from above ; C, from below. 
ast., facet for astragalus ; calc., facet for calcaneum ; mt., mt. 4, mt. 5, facets for the metatarsals ; 
nav., facets for navicular. | nat. size. 
The form of the bone which is regarded as the cuhoid of this animal is shown in 
text-fig. 34. The proximal surface (ast.), which articulates with the outer third of 
the distal end of the astragalus, is nearly flat and almost circular in outline, only the 
navicular border being somewhat flattened. Connected with the postcro-extcrnal 
angle of the astragalar surface is a small facet (calc.), looking backwards, outwards, 
and upwards, for articulation with the calcaneum. The inner face of the bone bears 
two small facets (nav.) for union with the navicular, one on the upper border, the 
other on the lower, the two being separated by a deep groove. The distal surface* 
is gently convex ; in front it is slightly bilobate {mt. 4, mt. 5) and clearly 
articulated with two metatarsals ; and the postcro-internal angle is produced into a 
blunt tuberosity. 
