578 



UNGULATA. 



point of union. In the lower molars the inner tubercles usually 

 unite to form an inner wall. The lower molars are usually 

 narrower than the upper. 



The tubercles are generally short (brachyodont) but in the 

 higher forms they lengthen and the spaces between them become 

 filled in with cement. In some of the ruminants (Bovidae) the 

 crowns become as long as in the Perissodactyla : this is the hyp- 

 sodont condition. 



The skull in the primitive extinct forms recalls that of the 

 Carnivora. In the higher forms the face becomes lengthened. 

 The lacrymals extend on to the face. 



In the more primitive forms the odontoid process is conical ; 

 with increased specialisation it tends to become spout-shaped. 



But this as in the 

 case of Agriochoerus 

 (p. 585) does not 

 always hold. In the 

 hinder dorsal and in 

 the lumbar vertebrae 

 the prezygapophysis 

 frequently bends 

 round the postzyga- 

 pophysis of the pre- 

 ceding vertebra, so as 

 to overlap and articu- 

 late with its dorsal 

 surface. The scapula is generally without an acromion. 



The ulna and fibula are complete and distinct in some forms, 

 but there is a tendency to their reduction and fusion with the 

 radius and fibula. In the ruminants the fibula is only represented 

 by its lower end (malleolar bone), \vhich articulates with the 

 calcaneum. As already stated the digits tend to diminish in 

 number. There are five metacarpals in the Oreodontidae (Fig. 

 297 bis, A) and Anoplotheridae, but No. 1 is reduced. In all 

 others this digit has completely gone, but digits 2 and 5 generally 

 persist though reduced. Digits 3 and 4 always persist and in the 

 higher forms their metapodia are united into a cannon bone which 

 is generally grooved in front or slightly cleft below and always 

 contains two medullary cavities. 



In the artiodactyl manus with five metacarpals metacarpal 1 



FIG. 297. Selenodont dentition of an Oreodont Agrio- 

 choerus latifrons. The upper teeth are above in the 

 figure (after Scott from Zittel). m first molar. 



