i6o 



THE GNAWERS OR RODENTS. 



This animal is one of the few placental 

 mammals of Australia. 



Secondly, we mention, by way of transition 

 to the next family, mice with elongated hind- 

 legs. Some of these (Meriones) have almost 

 the habit of hares. They dwell in the steppes 

 round the Caspian Sea. Others (Gerbillus) 

 have a greater resemblance to the jumping- 



Fig. 218. — The Egyptian Jerboa [Dipus mauritanicus). 



hares, and occur in Africa. They have a long 

 tail without any terminal tuft. 



THE JERBOA FAMILY 



(DIPODIDA). 



This family is so sharply distinguished 

 from all other rodents by the structure of 

 their hind-legs that some naturalists have 

 constituted them a sub-order. Their whole 

 structure is adapted for jumping, and we find 

 resemblances in this structure on the one 

 hand to the kangaroos, and on the other 

 hand to the jumping-shrews among the Insec- 

 tivora. The fore-feet, although very much 

 reduced in size, are yet perfectly well formed ; 



the collar-bone is present, and there are 

 almost always five toes armed with shafp 

 claws. A single American genus (Jaculus), 

 which is found in the Arctic Regions round 

 Hudson's Bay, has only four' toes and a 

 rudimentary first digit with a flat nail. These 

 fore-feet are used in walking only while the 

 animal is feeding; as a rule the jerboa goes 



Fig. 219. — The Jumping-rabbit of Siberia (A lactaga jaculus). p. 162. 



only on two feet, and the fore-feet are com- 

 monly used only for conveying food to the 

 mouth or for burrowing. The hind -leg, 

 which when extended to its full length is 

 longer than the body, has a long and thick 

 shin-bone (tibia), while the fibula is rudi- 

 mentary or rather degraded. The ankle is 

 greatly reduced with the exception of the 

 heel-bone (calcaneum), on which is a long 

 projecting process serving as a lever for the 

 attachment of the tendons of the muscles 

 used in jumping. The metatarsus is almost 

 as long as the tibia, and shows the greatest 

 variety of structure. In Jaculus and the 

 jumping-hares, which have five or four toes 

 on the hind-feet, there is the same number 



