SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 97 



the deeply concave astragalar surface is narrower transversely. I have 

 not seen the entocuneiform, but Ameghino's figure ('94^, p. 270, fig. iiD) 

 shows it to be a large, irregular, scale-like bone, shorter and wider than 

 in Proterotherium. The mesocuneiform, which Ameghino has incorrectly 

 supposed to be a vestigial first metatarsal ('94(5, p. 270, explanation of 

 fig. 11), is extremely small and even more reduced than in the genus last 

 named. The very large ectocuneiform has a greater proximo-distal length 

 in proportion to its width than that of Proterothermm, and the distal facet 

 for mt. Ill differs in being L-shaped, one arm extending across the dorsal 

 half and the other along the tibial side ; in the angle between the two arms 

 is a roughened, non-articular surface, thus making some approximation to 

 the condition seen in the horses : on the plantar side is a small facet for 

 the plantar hook of mt. III. 



The cuboid differs in several respects from that of Proterothermm ; the 

 plantar hook projects more prominently backward, but does not descend 

 so far distally, and on its tibial side is a long, narrow facet for the plantar 

 hook of mt. Ill, a facet which has not been observed in any of the tri- 

 dactyl genera of the family : the facet for mt. Ill, though still oblique, is 

 more distal than lateral, and that for mt. IV, which is very much smaller, 

 is shifted so as to be behind the surface for mt. III. 



In the horses the tarsus forms a nearly closed ring, the entocuneiform 

 and cuboid almost meeting on the plantar side, while in Thoatherhim this 

 tendency is but slightly indicated in the articulation of the cuboid with the 

 plantar hook of mt. III. 



The metatarsus, like the metacarpus, consists of one functional member, 

 mt. Ill, while mt. II and IV are reduced to mere nodular vestiges ; these 

 vestiges are attached laterally to mt. Ill, though mt. IV is somewhat 

 shifted to the plantar side, while in the horse the attachment of both splints 

 is entirely plantar. Metatarsal II, as in Proterothermm, articulates with all 

 three cuneiforms, excluding mt. Ill from contact with the mesocuneiform. 



According to Ameghino's figures ('94(5, pp. 271, 273, figs. 12, 13) meta- 

 tarsal III is of the same length as the corresponding metacarpal ; it is rela- 

 tively much more elongate and slender than in Proterothermm and of a some- 

 what different shape, the head being broader with reference to the shaft, 

 which is much more nearly cylindrical and less compressed planto-dorsally ; 

 the cuboid facet presents rather less laterally and more proximally, and 

 that for the ectocuneiform is interrupted by a large non-articular surface. 



