RODENTIA. 



17 



swimming quadrupeds, it is disproportionately large and 

 flat. The five digits are well developed, but the third and 

 fourth are considerably longer and stouter than the others. 

 The head of the fifth metatarsal is articulated to the outer 

 side of the fourth metatarsal, and not directly to the 

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

 large sesamoid bone on the tibial side of the tarsus, articu- 

 lating with the astragalus, navicular, and 

 internal cuneiform. The tuberosity of 

 the calcaneum is long and obliquely 

 compressed. 



The functional digits in other Ro- 

 dents may be five, as in the Rats, Por- 

 cupines, 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 elongated 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. 117), 

 which at first sight much 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 there 

 is a small fifth digit. 



Fig. 117 —Bones of right 

 pes of Jerboa [Dipus 

 a'Syptins), |. 



