52 TH1-: MAMMALIA OF THE UIXTA FORMATION. 



fourth are slightly smalioi- ; the unciform facet indicates that me. v (missing in this 

 collection) had ahout the same relative size as in the tapir. 



The tars'us. The astragalus rests by a rather nai-row facet upon the cuboid ; 

 this facet is equally broad posterioi'ly and anteriorly, as contrasted with the Hyra- 

 codon and equine series on the one hand and the Palicosyops series on the other. The 

 arrangement of the calcaneal facets is shown in Diag. 12. The calcaneum has a long 

 slender tuber expanding into a knob. The cuboid has its vertical diameter esceed- 

 ing the ti'ansverse, unlike all the more recent rhinoceroses. The articulation of the 

 third and fouith metatarsals with the ectocuneiform and cuboid is of the " plane 

 serial" type, but mt. ii abuts against the ectocuneiform. 



B. — In Ajyhelops the carpus and tarsus are very bi-oad and spreading and show 

 the extreme of diplarthrism. In both, the metapodial articulation is of the primitive 

 " altei-nating '' type. The toes sjii-ead widely, being reduced to three in each foot. 



Carpus (see Diagram 8). The scaphoid is very broad, covering the whole ante- 

 rior surface of the magnum, while the lunar rests anterioi'ly upon the unciform alone, 

 posteriorly upon the pivotal jDrocess of the magnum. A remarkable feature of this 

 bone is its unique articulation with the magnum hook posteriorly. The great devel- 

 opment of the middle toe and relatively increased size of the magnum, renders the 

 scapho-magnum surface broader than the lunar-unciform. Facets for the trapezium 

 are seen ui)on mc. ii and the trapezoid. The trapezoid and magnum facets of the 

 scaphoid are subequal and hoi'izontal, directly transmitting the weight. Mc. ii abuts 

 proximally against the magnum; mc. iii is much larger than the lateral metacarpals 

 and has a very broad unciform facet. There is no trace of a facet for mc. v. 



This type of carpus is precisely similar to that of R. unicornis and Ii. hioornis, 

 as figured by De Blaiuville. 



Tarsus. The astragalus is broad and low with a shallow trochlea, and broad 

 cuboidal facet, which is as large as that upon the calcaneum ; a characteristic feature 

 is the ental extension beyond the three distinct calcaneal facets. The calcaneum is 

 chai-acterized by a short tuber caleis and very slender sustentaculum; the three astra- 

 galar facets are sepai'ate and rather small. The transverse diameter of the cuboid is 

 greater than the vertical. The mesocuneiform is much reduced. The mts. ii and iv 

 have lateral fiicets upon the ectocuneiform and cuboid respectively of the "alternat- 

 ing" type. 



This type of tarsus is found in H. unicornis and M. hicornis, which are, however, 

 less broad and spreading. These recent genera differ from Aphelups and resemble 

 Acerat/wrium in the mode of metapodial articulation, the mt. iii not possessing a 

 cuboidal facet. In fact the cuboidal facet upon mt. iii in Aplielops is apparently unique. 



