able types Bathmodon, UintatJierium, and Uohasileus. These genera 

 contradict in several particulars the characters usually assigned to the 

 Proboscidia, while they agree with them in others, and they thus pre- 

 sent the problem of classification, which will ever recur so long as addi- 

 tions to our knowledge of the life of the past continue to be made. This 

 l^roblem is simply the question as to what characters shall be retained 

 as definitive of natural divisions, on the discovery of intermediate 

 forms. As our system is an expression of the possession of structural 

 characters, our higher groups or orders are naturally expressions of the 

 existence of the more comprehensive characters, or those present through 

 the most extended series of species. Hence we believe them to be also 

 those assumed earliest in time. 



In the case of the Ungulata^ the structure of the feet seems to define 

 the greatest range of the species. Thus the Artiodactyla and Perisso- 

 dactyla are digitigrade or unguligrade, while the Proboscidia are 

 l^lantigrade. The first order exhibits the equal development of the third 

 and fourth toes ; the second of the third toe, while in the Proboscidia 

 the structure is like the last, with more numerous digits. But this 

 order differs from both the preceding in the relations of the ulna and 

 radius. In Artiodactyla and Perissodactyla the ulna diminishes greatly 

 distally and presents but a small carpal articular surface obliquely be- 

 hind that of the much larger radius. In Proboscidia the ulna expanding 

 presents the larger articulation with the carpus, and the radius crosses it 

 obliquely, and presents its articular face alongside of the ulnar. 



The characters of the three orders may be thus stated : 



Proboscidia. * 



Feet plantigrade but elevated behind by a plantar pad. Toes uumer- 

 Ofls, short, the middle (3d) largest. Hind limb with knee free from the 

 body; tibia without spine; astragalus flat, not produced anteriorly. 

 Fore limb with well-developed ulna articulating extensively with the 

 carpus alongside of the smaller radius, which crosses it obliquely. 



Perissodactyla. 



Feet digitigrade, with a hock -joint. Toes reduced in number, the 

 third largest. Hind limb with knee inclosed in integument of body; 

 femur with third trochanter; tibia with spine. Astragalus with pulley- 

 shaped articular face for tibia and anterior prolongation. Fore limb 

 with ulna reduced ; its carpal surface smaller than that of the radius, 

 which supports the foot in front of the ulna. 



Artiodactyla. 



Feet digitigrade or unguligrade. Toes reduced, the third and fourth 

 principally and equally developed. Hind limb with knee api)lied to the 

 side of the body, and elevated hock; femur without third trochanter; 

 tibia with large spine. Astragalus with both inferior and anterior 

 pulley-shaped surfaces. Ulna much reduced distally, behind the radius, 

 which includes almost the whole of the carpal articulation. 



This arrangement violates previous views less than any other that 

 would recognize the primary characters of the Eobasileus. The difficulty 

 of determining the limits of tho, two first-named orders is partially 



* Tliese cliaracters have been mostly given Tby Prof. Gill, 1. c. 



