palaeontologists of Haydeii's geological surveys of 1871-2, of the remark- 

 able types Bathmodon, UintatJierium, and Eobasileus. These genera con- 

 tradict in several particulars the characters usually assigned to the Pro- 

 boscidia, while they agree with them in others, and they thus present the 

 problem of classification, which will ever recur so long as additions to 

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

 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 ex- 

 tended 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 nearly 

 plantigrade. 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 behind that of 

 the much larger radius. In Proboscidia the ulna presents the larger 

 articulation with the carpus, and the radius crosses it obliquely, and pre- 

 sents its articular face alongside of the uliiar. 



The characters of the three orders may be thus stated : 



PKOBOSCIDIA. * 



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

 rous, 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 

 along side 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 enclosed 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 sup- 

 ports 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 trochanter ; 



* These characters have been mostly given by Prof. Gill, 1. c. 



