168 UNGULATA order vii 



may inclose curved pits. The inferior molars are distinguished from those 

 above by their narrower breadth. In the bunodont forms the four cones 

 remain conical and generally distinct ; in selenodont types the external 

 cones are V-shaped or crescentic and the opening is directed inward. Their 

 usually compressed internal cusps as a rule meet, thus forming an internal 

 serrated crest. The posterior lower molar nearly always exhibits a fifth 

 unpaired cone or crescent, while the posterior upper molar is frequently 

 somewhat larger and never smaller than the preceding molars. Complete 

 agreement between molars and premolars never occurs in the Artiodadyla, 

 even though in individual forms of the most modern types (Dicotyles) and 

 in the terminal members of some extinct series (Dichodon, AgriocJwerus) 

 this condition appears to be approximated. The artiodactyl dentition is 

 typically heterodont, and the structure of the premolars is considerably 

 less complex than that of the molars. Sometimes the last premolar has 

 the appearance of a molar ; much more often, howevei^, it is similar to half 

 a molar. 



The milk dentition contains in concenti'ated essence the elements of 

 the permanent dentition. Incisors and canines deviate only slightly from 

 their successors ; on the other hand, the milk cheek teeth present greater 

 diversity than in the perissodactyls. The posterior upper milk tooth has 

 the form and structure of a true molar. The last inferior milk tooth closely 

 resembles the third molar, yet does not consist of the typical four cusps 

 or crescents and a talonid, but the third pair of cusps or crescents is here on 

 the anterior end of the tooth. In general, however, the anterior milk teeth 

 bear more resemblance to the premolars than to the molars. 



The scapula is narrow and triangular in form. The clavicle is absent 

 except in the more primitive families. The humerus is in the more specialised 

 forms shorter than the forearm, and longer only in some of the more primitive. 

 The radius and ulna either remain distinct or the distal portion of the latter, 

 sometimes also the proximal, is completely fused with the radius. The 

 carpus contains the same elements as seen in the perissodactyls and shows 

 the same lateral displacement of the distal series ; hence the magnum and 

 unciform alwa3^s support two ossicles of the proximal row. In more specialised 

 selenodonts the fusion of the magnum and trapezoid, and often the complete 

 atrophj'^ of the trapezium, occur. 



Five metacarpals are thus far known only in Agriochoeridae and 

 Anihrarotlieriidae (Fig. 226, A). The first metacarpal, however, is here very 

 small, vestigial, and onlj'' in some Agriochoeridae carries short phalanges. In 

 all the most primitive forms as well as in all existing Suidae and Hippo- 

 potamidae, the four remaining metacarpals are distinct. In the ruminants, 

 the lateral metacarpals are much reduced, and their respective digits 

 either hang free as short terminal claws or are completely atrophied. 

 If the lateral metapodials become styloid or entirely disappear, the median 

 ones generally show a tendency to coalesce. The "cannon bone" thus 

 formed always has, however, two medullary cavities, the distal end remains 

 cleft and exhibits two articular surfaces. The point of union is indicated 

 externally by a furrow on the anterior side of the cannon bone. Mobility 

 and speed depend entirely on the length of the metapodials. In the fleet- 

 footed ruminants, therefore, these are long and slender, but in the clumsy 

 hippopotamus, pig and their allies, they ai'e short and compact. The 



