THE MAMMALIA OF THE UIXTA FORMATION. 537 



series acquired a posterior navicular facet. The reduction of the entocuneiform fol- 

 lowed that of mt. i. The lil'th digit was much more rapidly reduced than in the 

 manus. Independently of these reductions, mt. ii and mt. in formed oblique sup- 

 porting facets with the ectocuneiform and cuboid respectively analogous to the 

 "alternating " metacarpal articulations, l)ut the metatarsal articulations were much 

 more variable than the metacai'pal. As a general rule metatarsal displacement anJ 

 growth was to the ectal side, the exception being in the Equidio, where mt. in 

 extended rapidly over the mesocuneifoi-m. 



The inetajjodials. In all the perissodactyls, primiti\e artiodactyls, and inadap- 

 tively reduced artiodactyls we iind, as above stated, but oue alternating type 

 of metacarpo-carjjal articulation ; the adaptively reduced artiodactyls develoj) a 

 second tyi'e of metacarpal articulation. In the jjes, beginning with the "serial" 

 type of Phenacodus, we find that genera tending to nionodactylism develop what 

 may be called the " p 1 a n e - s e r i a 1" type, in which there is for a period no lateral 

 spreading (J/i/raco(/ieriu7n). The "reverse" type is exemplified in AjjJielojys and 

 T. iiulicus, in which the second and fourth metatarsals both acquire ectocuneiform 

 facets. The alternating type is developed in the Titanotheriam tarsus and some 

 other Diplarthra precisely as iu the caipus. Each of these types has, of course, its 

 functional significance. 



"We may now consider some of the special characteristics of the manus and pes 

 in the five side lines. While observing many features which point back to the com- 

 mon taxeopod stem form, there are few or none which indicate any nearer mutual 

 affinity than this. 



TI. THE HYEACOIDEA, AMBLYPODA, PROBOSCIDIA, Etc. 



1. IIyuacoidea. 



An impoitant fact is brought out by the compari.son of the feet of /////".' and 

 Detulvohijrax. Tlie manus and pes are absolutely plantigrade. In both, the caipals 

 are arranged in strictly serial order, as described by Cuvier and Cope, although the 

 metacarpals .show decided lateral displacement. But in the tarsus of Ifi/r<ix the 

 calcaneum rests exclusively upon the cuboid, and, as in riiradcodax, the caleaneo- 

 cuboidal articulation is actually in .some ca.ses below the lc\ cl of the astragalo-navicu- 

 lai-.* In i'e/i(//o////?Y/a; (//. arWe?/s, Princeton Mu.seum), also from the Cape, the 



* Tlicso rpliilinn.4 witc foiiml liy rcmovinjt llio skin nnd fascia from llir fi>ol of a ilricd <i|>oi-imcn in the Co|«" Col- 

 li'flinn. I)c Bliilnvillc'!) flftnn-- nf //. r>//irn*u nml // tyriarun !«li<iw tln,'.»«' lii<x-ls <(ii llu' •^uni.- Irvol. 



