SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 95 



thick. It is shield-shaped, tapering to a blunt point at the distal end. 

 The anterior face is somewhat rugose and the surface for the rotular 

 groove of the femur is quite small and obscurely divided into two facets 

 by a low intercondylar ridge. 



The tibia (PI. XII, figs. 13, i^a) is relatively longer and more slender 

 than in Pyoterothermm and slightly exceeds the femur in length, exclusive 

 of the great trochanter ; the two parts of the bifid spine are even more 

 widely separated than in the latter genus and the facets for the femoral 

 condyles are of more unequal size, the external one being relatively larger ; 

 the cnemial crest is very prominent and heavy, but short, speedily dying 

 away upon the shaft, and the sulcus for the extensor tendon is very 

 deep and conspicuous ; the proximal fibular facet is extremely small. 'The 

 shaft is slender and has a distinct lateral sigmoid curvature. The distal 

 end is narrow, the antero-posterior diameter exceeding the transverse ; the 

 distal fibular facet is very small and divided into two parts, one lateral 

 and the other distal, the tibia extending over the fibula in a manner not 

 seen in any of the preceding genera of the family, somewhat like that 

 characteristic of the ruminant artiodactyls ; the internal malleolus is much 

 reduced and of little functional importance. The astragalar surface is much 

 as in Proterotherhmi, but with certain differences : the fossa for the inner 

 condyle of the astragalus is much shallower and the internal border much 

 lower, the posterior intercondylar tongue is longer and narrower, while 

 the facet on the anterior tongue for the pit on the neck of the astragalus 

 is more distal. In certain individuals the astragalar surface is invaded 

 from the inner side by a narrow, non-articular strip, which passes across 

 the intercondylar ridge, but not the outer condyle. This sulcus, the dis- 

 tinctness of which varies much, is not present in any specimen of Protero- 

 therhim which I have examined. 



In marked contrast with the distal end of the tibia in Tlioatherhwi is 

 that of the horse, with its broad, antero-posteriorly compressed shape, 

 deeply concave facets for the astragalus, and very large and prominent 

 internal malleolus, while the fibula is firmly ankylosed with the tibia: 

 this structure is much better adapted to prevent dislocation than is the 

 arrangement found in Thoathermm, in which indeed the structure would 

 seem to be inadequate. 



The fibula (PI. XII, fig. 13) is much reduced and very slender, but 

 shows no tendency to coalesce with the tibia ; the shaft is irregular and 



