XIX.] 



RODENTIA. 



349 



.11 



the cuboid. The middle cuneiform is very small. There is 

 a large sesamoid bone on the tibial side of the tarsus, 

 articulating with the astragalus, navicular, and internal 

 cuneiform. 1 The tuberosity of the calcaneum is long and 

 obliquely compressed. 



The functional digits in other Rodents may be five, as 

 in the Rats, Porcupines, and Squirrels ; 

 or the hallux may be suppressed, as in 

 the Hares ; and occasionally the fifth 

 digit is also wanting, . reducing the 

 number to three, as in the Capybara, 

 Viscacha, and Agouti. The last-named 

 animal has the three metatarsals elon- 

 gated and closely pressed together, 

 and all the digits with short subequal 

 phalanges. A still further modification 

 of the same type leads to the singular 

 condition of pes met with in the 

 Jerboas (genus Dipus, see Fig. 125), 

 which at first sight resembles that of 

 a bird. The three metatarsals are 

 ankylosed together to form a single 

 bone, which supports the three separate 

 short digits, each with three phalanges. 

 These alone are applied to the ground, 

 the tarsus and long metatarsal segment 

 being raised almost vertically. The hallux is wanting or 

 rudimentary, but in some species (Alactagci] there is a small 

 fifth digit. 



All the animals of the order UNGULATA are digitigrade, 



In Cercolabes and Erethizon there is also developed a claw-like bone, 

 articulating with the " sesamoid." G. Baur, " American Naturalist," 

 1885. 



k 



FIG. 125. Bones of right 

 pes of Jerboa (Dipus 

 cegyptins), \. 



