SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 1 39 



asymmetrical, owing to the larger size and greater prominence of the ex- 

 ternal condyle ; the neck is moderately elongate and bears, just below the 

 trochlea, a deep fossa, which receives the anterior tongue of the tibia, when 

 the foot is in extreme flexion on the leg. The head, which is depressed 

 and flattened in the dorso-plantar direction, is broad and quite strongly 

 convex ; it articulates only with the navicular and is widely removed from 

 any contact with the cuboid. On the plantar side are two facets for the 

 calcaneum, an external one, which is rather small, deeply concave and 

 invaded by a sulcus, so as to be almost divided into two parts ; the sus- 

 tentacular facet, which is narrow and quite widely separate from the exter- 

 nal facet, is almost plane. 



In Macrmichenia the astragalus has a broader and less deeply grooved 

 trochlea, a shorter neck, without fossa for the tibial tongue, and a less 

 convex head. The two plantar facets are much more nearly approxi- 

 mated, and the external one is larger and is not invaded by a sulcus. 



The calcaneum of Theosodon is very elongate, especially the tuber, 

 which contracts toward the free end, where it again expands in a club- 

 shaped thickening, but has no tendinal groove. The fibular facet, though 

 narrow, is large, conspicuous, strongly convex ; the external astragalar 

 facet is in two parts, which meet at an open angle. The sustentaculum is 

 large and prominent and bears a plane, oval facet for the astragalus. The 

 distal end is occupied by the extensive, twisted-looking surface for the 

 cuboid, which is obliquely concave both transversely and planto-dorsally ; 

 there is no facet for the astragalus on the inner side of the distal end, the 

 two bones being separated at this point by a narrow space. 



In Macrmichenia the calcaneum has a relatively shorter and heavier 

 tuber and a smaller facet for the fibula. 



The navicular (of Theosodon) is rather narrow, but of relatively con- 

 siderable proximo-distal height, especially the external half of it. From the 

 plantar face is given off an extremely large and prominent process or 

 hook (see PI. XX, fig. 8) which, though compressed laterally, is yet very 

 heavy. Proximally, the navicular bears quite a deep concavity, to receive 

 the head of the astragalus, and the distal end bears facets for the three 

 cuneiforms. That for the entocuneiform is small and obliquely lateral 

 rather than distal in position, that for the mesocuneiform is larger, while 

 the facet for the ectocuneiform is still larger and descends farther distally, 

 which gives an appearance as though a step were cut in the navicular to 

 receive the mesocuneiform. 



