26 PATAGONIAN EXPEDITIONS: PALEONTOLOGY. 



transversely, increasing in width toward the distal end ; for nearly half its 

 length, the shaft has on each side a broad, shallow depression, to receive 

 the lateral metacarpals ; on the dorsal side, near the radial border of the 

 proximal end, is a prominent tubercle for the attachment of the extensor 

 tendon. The distal trochlea is broad and very low proximo-distally, but 

 has a very prominent carina, which extends around the entire dorsal face 

 of the trochlea, quite as in the horses. 



Metacarpal IV is the counterpart of mc. II : proximally, it is overlapped 

 but little by the head of mc. Ill and articulates with the unciform by 

 means of a slightly convex facet. 



A vestige of metacarpal V is attached to the unciform and mc. IV, and 

 is a very small, almond-shaped bone, ending distally in a blunt point. 



The phalanges of the lateral digits are small, slender, laterally com- 

 pressed and resemble quite closely those of Protohippus : the unguals are 

 pointed and somewhat irregularly shaped. On the contrary, the phalanges 

 of the median digit are greatly elongated, together exceeding the meta- 

 carpal in length, and have a marked resemblance to those of the horses. 

 The proximal phalanx is especially long and is also broad, but is much 

 depressed palmo-dorsally : the proximal end is very broad and has a shal- 

 low, concave trochlea, deeply grooved in the median line to receive the 

 metacarpal carina, which, however, does not notch the dorsal border of 

 the trochlea. Below the proximal end the shaft at first narrows rapidly, then 

 more gradually, broadening again somewhat at the distal end : the distal 

 trochlea is very low and saddle-shaped, concave transversely and convex 

 palmo-dorsally. 



The second phalanx is short and broad ; the proximal end is quite thick 

 in the dorso-palmar dimension, but this thickness diminishes rapidly and 

 the distal end is strongly depressed : the proximal trochlea is a single 

 transverse groove, without such intercondylar ridge, or division into two 

 facets, as may be seen in the horses : the distal trochlea also lacks the 

 median groove of the equines and is more depressed. The ungual 

 phalanx is exceedingly like that of such three-toed equine genera as Pro- 

 tohippus ; it is broad, depressed, with rounded free border and conspicuous 

 median cleft at the distal end : the proximal trochlea is an undivided trans- 

 verse groove, and the lateral processes are moderately well developed : 

 the palmar surface is slightly concave, v/ith a deep crescentic concavity near 

 the proximal end. 



