574 CHORD AT A. 



Hence we find that gradational adaptive structure in living forms is 

 mainly due to "gradational" environments, and that in fossil forms it 

 is due to gradual change of environment. 



SUB-ORDER V. — ARTIODACTYLA. 



The Artiodactyla form a large branch or assemblage of 

 Ungulata, which in many respects show parallel evolution to 

 the Peris sodacty la. They follow, however, rather different 

 lines: — (i) The molar teeth are bunodont or selenodont* 

 (2) The dorso-lumbar vertebrae are nineteen. (3) The 

 femur has no third trochanter. (4) No alisphenoid canal 

 and small nasals. (5) The carpus and tarsus are alternate 

 and the toes are four or two ; the main axis is between the 

 third and fourth toes. (6) The stomach may be simple or 

 complex and the placenta diffuse or cotyledonary. 



One division of the Artiodactyla retain the bunodont 

 teeth {Bunodonta), only multiplying the number of the 

 tubercles, whilst the other division {Selenodonta) have the 

 tubercles twisted into crescents or curves and worn down, 

 thus producing the selenodont type. As in the Peris so- 

 dactyla, there is the addition of cement and the heightening 

 of the crowns. 



The femur appears to have lost its third trochanter very 

 early in the history of this sub-order. The toes show the 

 same gradational reduction as in Perissodactyla, but on a 

 different plan. The third and fourth toes are always equal 

 and larger than the second and fifth. These latter are 

 hoofed and touch the ground in pigs, but are greatly re- 

 duced in sheep and oxen and disappear altogether in the 

 camel. It follows from the main axis passing between the 

 third and fourth toes that the cuboid and ectocuneiform 

 tend to be more or less equally developed, and that the 

 astragalus articulates equally with the cuboid and navicular 

 whilst the fibula, or its distal end, still remains in articula- 

 tion with the calcaneum. The cuboid often fuses across 

 the middle line with the navicular. 



Family 1. — Hippopotamidae. — The Hippopotamus is confined to 

 the rivers of Africa. Its canines and incisors are large and grow from 

 persistent roots. The molars are of a slightly modified bunodont type 

 and each tubercle wears into a three-lobed crown. The stomach is 

 complex and the diet herbivorous. All four toes (first is absent) are 



