SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 93 



ing that a foot-structure of perissodactyl symmetry did not admit of the 

 inadaptive method of reduction. As a matter of fact, however, this 

 method has not been observed in any of the genuine Perissodactyla. 



Metacarpal II is a mere vestige and consists only of the proximal por- 

 tion of the original bone ; the head is relatively large, both transversely 

 and palmo-dorsally and quite extensively overlaps the head of mc. Ill, the 

 facet for which presents distally and covers the radial portion of mc. Ill 

 for its whole dorso-palmar diameter. The facet for the trapezoid occupies 

 the entire proximal surface ; that for the trapezium, which is relatively 

 much smaller than in Proterotheriuni, is obliquely lateral in position. On 

 the ulnar side is a facet for the magnum, and thus all the original connec- 

 tions are retained. — Metacarpal III, the only functional one of the series, is 

 relatively longer and much more slender than that of Proterotiieriitm. 

 The proximal end is considerably expanded and is of nearly the same 

 dimensions as in the smaller species of Proterotherhini, but the following 

 differences should be noted : (i) the facet overlapped by the head of mc. 

 Ill is less oblique and more directly proximal ; (2) the facet for the mag- 

 num is less simply concave transversely, becoming convex toward the ulnar 

 side ; (3) the unciform surface is much smaller and presents more prox- 

 imally, less laterally ; (4) the shaft is longer, more slender and rounded, 

 less compressed palmo-dorsally. As in all of the members of this family, 

 the carpal connections of mc. Ill are primitive, as the bone is excluded 

 from contact with the trapezoid and touches the unciform only by a mi- 

 nute lateral facet. Although the functional metacarpal bears such a 

 close general resemblance to that of the modern horse, it differs notably 

 from the latter in mode of articulation with the carpus. 



Mc. IV is a minute rudiment, measuring only i cm. in extreme length, 

 and is a compressed, scale-like bone, tapering distally to a blunt point. 

 These minute splint bones differ from those of the horse, not only in their 

 very much smaller size, but also in their more lateral position : mc. II 

 still retains its connection with the magnum, and excludes mc. Ill from 

 the trapezoid 



The proximal phalanx is much narrower and less depressed than that 

 of Proterotherium and its distal trochlea is more grooved in the median 

 line ; the second phalanx also is much more slender, while the ungual is 

 longer and narrower, more concave on the palmar face and with much 

 more prominent median dorsal beak for the trochlea of the second phalanx. 



