522 THE VERTEBRATE SKELETON. 



common integument. This condition is known as syndactylism, 

 and its effect is to produce the appearance of one toe with two 

 claws. In the Kangaroos (Macropodidae) the pes is very long 

 and narrow, owing to the elongation of the metacarpals. The 

 fourth digit is greatly developed, the fifth moderately so, 

 while the hallux is absent, and the second and third digits 

 are very small. The Peramelidae have the foot constructed 

 on the same plan as in the Kangaroos, and in one genus 

 Choeropus the same type of foot is carried to a greater 

 extreme than even in the Kangaroos. Thus the fourth digit 

 is enormously developed, the second and third are small, and 

 the fifth smaller still, while the hallux is absent. In the 

 Phalangers and Koalas though the second and third toes are 

 very slender, the hallux is well developed and opposable. 



EDENTATA. In the Sloths the pes much resembles the 

 manus, being long and narrow, but in both genera the second, 

 third and fourth digits are well developed. Most of the other 

 Edentates have a but little modified pes with the normal 

 number of tarsal bones and the complete series of digits. In 

 Cycloturus however the hallux is vestigial and it is absent 

 in Glyptodonts. Megatherium has a greatly modified pes, the 

 hallux is absent, and the second digit vestigial, while the 

 third is very large, having an enormous ungual phalanx. The 

 calcaneum too is abnormally large. 



No trace of the pes occurs in either SIRENIA or CETACEA. 



In the UNGULATA the pes like the manus is subject to 

 much variation and is of great morphological importance. 



In the UNGULATA VERA the pes is never plantigrade and 

 never has more than four digits, the hallux being absent. 

 The cuboid always articulates with the astragalus, and the 

 tarsal bones strongly interlock. As was the case also with the 

 manus, the pes is formed on two well-marked types character- 

 istic respectively of the Artiodactyla and Perissodactyla. 



ARTIODACTYLA. Just as in the manus, the third and 

 fourth digits are well and subequally developed ; their ungual 



