SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 99 



very significant differences, which preclude any close association of the 

 two families, (i) In Thoatherium the tarsus is fundamentally different; 

 [a] the calcaneum retains its primitive connection with the fibula and is 

 nearly intermediate in character between the artiodactyl and the perisso- 

 dactyl type (the fibulo-calcaneal articulation of the horses has been sec- 

 ondarily acquired and is of an entirely different nature); [b] the astraga- 

 lus is long and narrow and has a depressed-convex head for the navic- 

 ular, which recalls that of the Condylarthra, and is widely removed from 

 the cuboid, while its facet for the sustentaculum is very artiodactyl in 

 character ; [c] the navicular and ectocuneiform retain their relatively great 

 proximo-distal length, very different from the low, broad, disc-like bones 

 of the horse ; [d) the cuboid and ectocuneiform are well separated on the 

 plantar side. 



(2) The mode of digital reduction is inadaptive, mt. Ill being shut off 

 from the greatly reduced mesocuneiform, which, though almost atrophied, 

 is not ankylosed with either of the adjoining bones ; the cuboid surface is 

 oblique, as much lateral as proximal, while the ectocuneiform surface is much 

 less extensive than in the horse. In the horses, mt. Ill articulates with 

 all the distal tarsal bones by surfaces which present proximally and which 

 are so arranged as to cover almost all the periphery of the proximal end, 

 and there is no plantar hook- In Thoatherium these facets form less than 

 a semicircle, though this is, to a certain extent, compensated for by the 

 great hook which arises from the plantar side and articulates with the 

 cuboid and ectocuneiform. In brief, the Litopterna have solved the 

 problem of monodactylism in a manner which is distinctly dift"erent from 

 that followed by the Perissodactyla. 



Restoration (PI. XV). — The lightness and grace of Thoatheriztm are 

 conspicuous features. The head is smaller and lighter, proportionately 

 somewhat longer \h2in'\n Diadiap horns (see Plate I), but of more slender 

 shape and much shallower dorso-ventrally. The neck is strikingly short, 

 which is made all the more conspicuous by the elongation of the limbs. 

 The length of the trunk remains largely conjectural and the appearance of 

 the skeleton, as given in Plate XV, might be materially changed by the 

 discovery of a complete vertebral column. From the evidence now avail- 

 able, it seems very likely that the trunk was intermediate in relative length 

 between the artiodactyl and the perissodactyl type, a little longer than in 

 the former and slightly shorter than in the latter. The remains preserved 



