170 



UNGULATA 



ORDER Vir 



with the calcaneum. On its anterior side, above, the calcaneum (Fig. 227) 

 has a rounded facet {p) for the fibula. In the astragalus (Fig. 228) the 

 superior and anterior articular surfaces (trochlea) for the tibia are deeply 

 excavated {t) ; the distal portion («, cub) resting on the navicular and cuboid 

 is not truncate as in the perissodactyls, but likewise forms a convex articular 

 face, which passes into a large rounded facet [cal) on the posterior side. In 

 more primitive tetradactyl forms with well-developed lateral digits, the tarsal 

 elements remain distinct. In the ruminants, as a rule, a fusion of the cuboid 

 and navicular takes place, the second and third cuneiforms likewise coalesce, 

 and sometimes even the cuboid, navicular and two cuneiforms unite into a 

 single bone. 



In the pes as in the manus there is a gradual reduction or atrophy of the 

 lateral metapodials and digits. The first metatarsal is always vestigial, this 

 remnant usually representing the phalanges also. Through the partial 

 atrophy of one lateral digit, the hind - foot becomes tridactyl in the 

 Anoplotheriklae and Dicotijles. The two median metatarsals fuse into a 



Fio. 227. 



Calcaneum of Aaoplotherium 

 commune Cuvier, anterior aspect. 

 a<, Articul.ir facet for astragalus ; 

 cub, for cuboul ; p, for tibula. 

 (After Gaudry.) 



Fig. 228. 



Astragalus of Helladntherinm duvernoyi Gaudry. A, Anterior 

 aspect ; B, posterior aspect; a, free inner border; c, c', lateral facet 

 for calcaneum ; cnl, posterior facet for calcaneum ; cub, facet for 

 cuboid ; n, facet for navicular ; t, trochlea for tibia. (After Gaudry.) 



cannon bone, this coalescence sometimes occurring in the pes even though 

 the met ipodials of the manus still remain distinct (Hyaemoschns, Dicotijles). 

 In general, reduction and co-ossification appear earlier and show more 

 distinctly in the pes than in the manus. In the " inadaptive " development 

 of the extremities, the metatarsals maintain their position among the corre- 

 sponding tarsal elements. In the "adaptive" reduction the proximal ends of 

 the principal metatarsals expand and push the lateral metapodials outward 

 and backward. The digits of both extremities are similar in structure. The 

 terminal phalanges are triangular, encased in hoofs, and only in the 

 AnopUth'riidae and Agriochoerns are olawlike. 



For a phylogenetic understanding of the modification of the extremities 

 in the artiodactyls, it is interesting to note that in the more specialised 

 forms (Cavkoruia) the co-ossified bones of the adult condition are separately 

 developed in the embryo, and thus correspond to the forms geologically 

 older. The developmental stages that apparent^ so widely separate the 

 clumsy short tetradactyl foot of Hippopotamus and the slender elongated 

 double-hoofed extremities of the ruminants, are brought into closest connection 

 through numerous intermediate fossil forms, and therefore up to a certain 



