58 



TEETIAET YERTEBEATA OF THE FATtlM. 



This bone differs widely from the navicular of Elephas 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 prominence 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 Coryphodon 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. 



ale. 



nav. 



ml. 



B, 



K 2 



calc 



nau 



mt.S. 



Eight cuboid of Arsinoitherium zitteli : A, navicular face ; B, from above ; C, from below. 



agt., 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 cuboid 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 postero-external 

 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 postero-internal angle is produced into a 

 blunt tuberosity. 



