the mammalia of the uixta fokmation". g-dj 



2. The Equus Series. 



The equine series is the most complete known, and is well distingulslied from 

 the others by the fact that the eifects of lateral displacement in the carpus, which are 

 well advanced in the Eocene membcn-s, although never extreme, are subsequently 

 counterbalanced, and practically lost sight of, in the growth of the bones in the 

 median line resulting from monodactylism. In the tarsus the displacement is very 

 limited and is not similarly retrogressive. It is surprising to note the constancy in 

 size and shajje of the small astragalo-cuboid articulation from the Wasatch llijraco- 

 theriurn to the recent horse; secondly, the early appearance of minor eijuine niMrks in 

 the manus and pes. The disjilacement or diplarthrism is thus limited in both feet. 

 The impoitant secondary characteristic is the backward rotation of the lateral ele- 

 ments of the carpus, tarsus and metapodials. 



Caupus. The distinctive feature is the fact that the scaphoid eaily gains a 

 somewhat broader foothold upon the magnum than the lunar has upon the unciform. 

 This may be attributed to the early enlargement of mc. iir, and ectal growth of the 

 magnum. The lunar in the Eocene members retains subequal magnum and uiieiluiiu 

 facets (as in Iseclulophuf^), but the carpus is laterally compressed. Ii: ilu- more 

 recent forms the magnum, by its rapid growth, extends beneath the lunar, and the 

 latter bone is thus brought back to its primitive position and rests in some species 

 exclusively upon the magnum. 



I/ijracothenum and Epiliiiypus. There is no marked dillerence betAveen the car- 

 pus of //. venticoUun and of E/)i/iippus, excepting the advanced reduction of mc. v. 

 In ^pifn'pjuis (Xo. 10,401, PI. XI, Fig. 4), the bones of the upper row are elongate. 

 The seal) ho id '•'' directed backwards and rests distally u[)on the trapezoid and 

 magnum, ])y sube([ual horizontal facets. The trapezium contact is very small. The 

 lunar has the high and slender pi-oportions seen in Triplopiix^ but ihi- ilisial facets 

 for the magnum and cui\eiform are subequal and incline obli(piely towards each other. 

 The cuneiform is narrow and slender. The trapezium was evidently small, articu- 

 lating with the scaphoid, trapezoid and mc. ii. The trapezoid is much Hattened. The 

 m agn u m supports the lunar anteriorly and posteriorly. The u n c i Co r ni has an 

 infero-lateral facet for the fifth metacarpal. The foot is practically tridactyl, jnc. v 

 being well retracted. The metacarpal articulation is of the a 1 1 c in a t i n g type. 



The carpus of Mcso/iijijms has never been described, mir thai nl' any American 

 species of Anrhilhnrium. Referring to Kowalevsky's Memoir (I 'I. 11. I"'ig. la), wc 

 observe that the mag u u m extends beneath three-fourths of the lunar and the same 

 proportion of the scaphoid, yet the metacarpal articulation i-< unaltered, mcs. ii and 



