5G2 GEOLOGICAL SURVEY OF THE TERRITORIES. 



able types BafJtmodon, UiniafJwrmm, and Eohasilens. These genera 

 contradict in several particulars the characters usually assigned to the 

 Prob(Ksei(lia, while they agree .with them in others, and they thus pre- 

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

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

 problem is sim})ly the question as to, what characters shall be retained 

 as detiuitive 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 Unrpdata, the structure of the feet seems to define 

 the greatest range of the species. Thus the Arflodactyla and Ferisso- 

 (lactyla are digitigrade or unguligrade, while the Prohoscidia are 

 plantigrade. The iirst order exhibits the equal development of the third 

 and lx)urth toes ; the second of the third toe, while in the Froboscidia 

 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 Ferlssodaciijla the ulna diminishes greatly 

 distally and presents but a small carpal articular surface obliquelj' be- 

 hind that of the much larger radius. In Froboscidia 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 : 



Froboscidia.* 



Feet plantigrade but elevated behind by a i^lantar pad. Toes numer- 

 ous, shtut, 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 boily ; 

 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 applied to the 

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

 tibia with large spine. Astragalus with both inferior and anterior 

 })ulley-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 the two first-namect orders is partially 



' These characters have been mostly given by Prof. Cjill, 1. c. 



