SCOTT : LITOPTERNA OF THE SANTA CRUZ BEDS. 59 



is rather more convex and that for the pisiform makes a more open angle 

 with it. The trapezium is an irregular, nodular bone ; it articulates prox- 

 imally with the scaphoid by a large, slightly convex surface, laterally with 

 the trapezoid and has an extensive oblique facet for the second metacarpal ; 

 the distal end is not bluntly pointed and rugose, as it is in Theosodori 

 and the Perissodactyla, but transversely truncated and smooth, as though 

 for the attachment of a vestigial first metacarpal, such as Ameghino has 

 described in Diadiaphorus ('94<5, p. 268). The trapezoid resembles that 

 of the latter genus, except in being relatively narrower, as is also the 

 magnum, of which the width but slightly exceeds the proximo-distal 

 height, and the facet for the lunar is slightly more raised and convex in 

 the dorso-palmar direction, while that for the scaphoid is more concave 

 transversely. I have seen no example of the unciform. 



The metacarpus (PL XI, figs. 4, 9, <^a\ like the other bones of the extrem- 

 ities, much resembles that of Diadiaphorus, but is rather more slender. 

 Metacarpal II is longer in proportion to III than in the last named genus, 

 but is of similar shape and has similar connections with the carpus. 

 Metacarpal III is somewhat more slender than in DiadiapJioriis, (though 

 the difference is not so great as it appears at first sight, because of the 

 smaller size of Proterotherittni) and broadens less to the distal end : the 

 tuberosity near the proximal end for the attachment of the extensor tendon 

 is less prominent and the distal carina is rather sharper. Metacarpal IV, 

 like II, is relatively longer than in Diadiaphonis, but otherwise similar. 

 I have seen no example of the vestigial mc. V, but its presence is demon- 

 strated by a minute facet on the head of mc. IV. 



The phalanges of the median digit (PL XI, figs. 9, 91^) are very like 

 those of Diadiaphorus and the proximal phalanx has almost the same pro- 

 portions as in that genus ; the second is of more uniform width, contract- 

 ing less toward the distal end, while the ungual is a little more depressed, 

 with less convex dorsal surface. Obviously, there is some mistake in 

 Ameghino's figure of P. intennixtum [sic] ('94^, p. 269, fig. 10) which is 

 apparently due to the association of phalanges of TJioatheriuni with the 

 metacarpals of Proterotherium. 



The pelvis (PL XII, fig. 2) has considerable resemblance to that of the 

 equine genus Mesohippus from the Oligocene of North America. The 

 ilium is short and has a short peduncle, which rapidly expands into the 

 very broad anterior plate ; this plate is obscurely divided by a shallow 



