1910.] Astragalus in Artiodactyla, etc. 457 



in Anisonchus (Matthew, 1897, p. 297, %. 11), Periptychus (Osborn, 1898, 

 p. 181) and Pantolambda (Osborn, 1898, p. 188). The evolution of the pes 

 in the Amblypoda has been described above (pp. 357-358). 



Artiodactyla. The derivation of the Artiodactyl astragahis from the tvpe 

 seen in the Mesonychid Creodonts was described above (p. 404). This was 

 from the first a strictly cursorial type, since motion was limited to the fore 

 and aft plane, not only by the sharp trochlear keels but also by the trans- 

 verse flattening and antero-posterior convexity of the navicular facet. The' 

 presence of the astragalo-cuboid facet in addition to the navicular facet gave 

 rise to Cope's term " Diplarthra." By false analogy with the interlocking 

 carpus, which was again falsely thought to be an entirely secondary arrange- 

 ment, the doubly unwarranted conclusions were reached that "diplarthrism" 

 is a wholly secondary acquirement in both Artiodactyls and Perissodactyls, 

 and further, that it is an indication of close relationship between those two 

 orders. As it appears to the writer, the Artiodactyls were essentially dip- 

 larthrous long before they became Artiodactyls, although of course the 

 astragalo-cuboid facet widened progressively. An essential feature of the 

 Artiodactyl astragalus is the very large flattened sustentacular facet on the 

 back, which is foreshadowed in the Mesonychidse. 



Perissodactyla. In this order the astragalo-cuboid contact may at first 

 have been almost or entirely absent; it was absent or at most exceedingly 

 narrow in all the Lower Eocene Perissodactyls; it widened progressively, 

 especially in Rhinoceroses and Titanotheres but never became as extensive 

 as in the Artiodactyls; the navicular facet generally became flatter; the 

 sustentacular facet formed a reversed L (j), a distal calcaneal facet also 

 developed (p. 395). 



2. The Calcaneum. 



In the Monotremes, which are entirely plantigrade, the tuber of the 

 calcaneum is pointed downward. In the most primitive Polyprotodont 

 Marsupials and Insectivores, the heel is slightly raised from the ground 

 and the calcaneum points backward and slightly downward. In the digiti- 

 grade and unguligrade mammals the tuber is elongate and points backward 

 and upward. 



