Hatcher: Little Known Fossil V^ertei'.ratks. 141 



ulates with the trapezium, trapezoid and nKigiiuni, covering the entire 

 upper surface of the first two, and only about one half the superior an- 

 terior surface of the magnum, thus contrasting strongly with some of 

 the later tridactyl Aceratheres, in which the scaphoid covers the entire 

 superior anterior surface of the magnum. The facet for the trapezium 

 is slightly concave, and has the general form of an elongated ellipse. 

 The articular facet for the trapezoid is concave transversely and con- 

 vex anteroposteriorly. Two rather sharp ridges separate it from the 

 facet for the trapezium on the one side and that for the magnum on 

 the other. The latter facet is triangular in outline, and gently con- 

 cave transversely. The radial facet of the lunar is strongly convex 

 anteroposteriorly, whiJe inferiorly this bone articulates with the mag- 

 num by an elongated, deeply concave facet, broad posteriorly, but 

 somewhat constricted anteriorly. With the unciform it has also an 

 extensive contact, ovate in outline and concave anteroposteriorly. 

 The pyramidal shows the usual superior facets for the ulna and pisi- 

 form, while inferiorly it articulates only with the external superior 

 surface of the unciform, occupying somewhat less than one half the 

 upper surface of that bone. The pisiform articulates about equally 

 with the pyramidal and ulna. The neck is much constricted, while 

 the tuberosity is greatly expanded, terminating superiorly in an ele- 

 vated process. I'he trapezium is the most diminutive bone in the 

 distal series of carpals. It articulates superiorly with the scaphoid and 

 exteriorly with the trapezoid, terminating inferiorly in a small pointed 

 process without articular surface. The trapezoid is a small bone with 

 superior and inferior facets, concave anteroposteriorly and convex 

 laterally. These articulate respectively with the scaphoid and meta- 

 carpal II. The magnum is comparatively large, and bears three 

 articular facets on its superior surface, one situated anteriorly and ex- 

 ternal, articulating with the scaphoid ; a second, narrow in front, but 

 expanding jjosteriorly, situated about midway between the external 

 and internal margins, and articulating with the lunar; the third is 

 placed anteriorly and internal, and articulates with the unciform. In- 

 feriorly the magnum articulates only with metacarpal III, while there 

 is also a lateral articulation between the magnum and metacarpal II. 

 The tuberosity is narrow and directed downward and inward. The 

 unciform is much expanded inferiorly and articulates by a continuous 

 facet with metacarpals V, IV, and the external one third of the proxi- 

 mal end of metacarpal III, and in addition it covers about one third 



