SUBORDER D ARTIODACTYLA 167 



from each ■ other, but which through numerous extinct intermediate forms 

 are brought into closest relations. The distinguishing character of the 

 artiodactyls is the even number of digits. The middle digits (III and 

 IV) are always equally developed, the lateral toes are more slender or 

 quite rudimentary, the first digit or pollex usually occurring only in some 

 few extinct forms {Agriochoeridae, Anthracotheriidae). The weight of the body 

 is borne by the two median digits, the axis of the extremities falling between 

 these. 



The artiodactyls embrace in part slender, long-limbed, in part clumsy, 

 short-limbed forms. In the more primitive types the skull recalls that of the 

 carnivores and perissodactyls, but in the specialised forms, through the 

 lengthening of the facial bones, the development of air-cells in the frontal 

 region, of horn-cores, horns, etc., the skull acquires a very distinctive 

 character. The lachrymals appear to be fairly expanded on the upper 

 sui"face of the skull, and in the ruminants frequently exhibit rather deep 

 depressions (lachrymal fossae) for the reception of sebaceous glands. The 

 frontals always enter into the formation of the brain-case, and sometimes 

 attain very large size. The mandible is usually long, slender, and low, with 

 an ascending coronoid process. 



The dentition is originally composed of forty-four teeth, which in the 

 more primitive forms are arranged in a continuous series. Through the 

 lengthening of the jaw or the suppression of the anterior premolars, and 

 sometimes of the upper canines also, diastemata arise between the front 

 teeth and the cheek teeth ; these are longest in the ruminants, where the 

 lower canine lies close to the incisors and assumes the form and functions 

 of a cutting tooth (except in Myotragus where the lower front teeth are 

 reduced to a single enlarged pair of incisors). The reduction, modification, 

 or suppression of the incisor and canine teeth begins in the upper jaw earlier 

 than in the mandible. Vestiges of superior incisors are still to be recognised 

 in the embryo of the sheep. 



In all older and more primitive forms, the cheek teeth are brachyodont ; 

 in the ruminants they are in part prismatic. The development of cement 

 as a rule also accompanies this deepening of the teeth. The crown of the 

 molars in the later forms is quadritubercular. The cusps are arranged 

 in opposite pairs. Originally the superior molars were quinquetubercular, 

 and the lower molars sexitubercular. But in the latter the paraconid and 

 metaconid were soon lost or reduced, or the paraconid fused with the 

 metaconid, while in the superior molars the anterior intermediate cusp 

 or protoconule was soon united with the protocone. The second inner 

 cusp of the upper molars is not the real hypocone, but the metaconule. 

 In Caenotherium and Dichobune the posterior half of the upper molars 

 shows three cusps, the innermost of which is, according to Stehlin, an acces- 

 sory one. 



Accessory pillars as well as strongly developed basal cingula occur in 

 many artiodactyls. If the tubercles remain conical, the dentition is buno- 

 dont ; if they assume a V-shaped or crescentic form, the dentition is 

 bunoselenodont or selenodont. In selenodont upper molars, the crescents 

 of the external cusps open outward and generally meet, forming at the 

 point of union an ectoloph with a vertical fold (mesostyle). 



The internal cusps may remain either conical or V-shaped, or the crescents 



