PERISSODACTYLA. 311 



Order 5. PERISSODACTYLA (ODD-TOED UNGULATES).* 



Large, Ungulates usually of unwieldy build ; the middle digit is more 

 developed than the others. The stomach is simple and the ccecum is 

 very large. The dentition is usually complete. 



In the earlier tertiary times the Ungulates were already a well- 

 defined group, the smaller species of which presented approxima- 

 tions to the Insectivora (Microchoerus) and Rodents. The Ungulata 

 are either herbivorous or omnivorous. The dentition is highly 

 differentiated ; the grinders are traversed by folds of enamel, with 

 transverse ridges and short tubercles, which are usually worn down 

 to an even, masticating surface. Large chisel-shaped incisors, which, 

 however, may fall out or in the lower jaw be completely absent, are 

 often present. There is always a gap between the incisors and the 

 praemolars. The canines are often absent, or only present in the 

 upper jaw, principally in the males, and then are transformed into 

 tusk-like weapons. Even when both upper and lower canines are 

 present, they have this significance, and are much larger in the 

 male sex. 



Among the many differences which the Ungulates present in 

 their whole organisation and mode of life, the difference in the 

 number of hoofs (which corresponds with that of the toes) was 

 held to have a special value, and accordingly Multiungulates, 

 Biungulates, and Uniungulates were distinguished as separate 

 orders. This division was, however, by no means a natural one, 

 since it led not only to the union of widely divergent forms as 

 Multiungulates, but also to the separation of the Uniungulates and 

 Biungulates from their near allies. The progress of palseontological 

 knowledge has shown that this division is untenable. Remains 

 of extinct forms, which partly fill up the gaps between members of 

 the supposed orders, have been discovered. Accordingly the order of 

 Multiungulates has been recently broken up, and two members of it 

 the Proboscidea and the Hyracoidea have been placed among the 

 Deciduata ; and further, two orders founded upon the odd or even 

 number of the toes a character which had already been used by 



* G. Cuvier, " Eecherches sur les ossements fossiles," Third edition, Paris, 

 1816. 



T. Rymer Jones, Article ' Pachydermata " in Todd's " Cyclopaedia," with 

 Supplement by F. Spencer Cobbold, 1859. 



W. Kowalevski, " Monographic des Genus Anthracotherion Cuv., unci 

 Versuch einer natiirlichen Classification der fossilen Hufthiere. Palgeonto- 

 graphica," 1873. 



