MAMMALS THAT GNAW. 109 



while the food is poked in with the other. It is obviously impossible for 

 the animal to pass food from the mouth to the pouches without the aid of its 

 fore-claws. The most remarkable thing connected with the use of the 

 pouches is the way they are emptied. The fore-feet are brought back simul- 

 taneously along the sides of the head until they reach a point opposite the 

 hinder end of the pouches ; they are then pressed firmly against the head 

 and carried rapidly forward. In this way the contents of the pouches are 

 promptly deposited in front of the animal. Sometimes several strokes are 

 necessary, I am not prepared to say that the animal cannot empty the 

 pouches by means of the delicate investing muscles ; but I have never seen 

 them emptied in any other way than that here described." 



According to American writers, the typical members of the family may be 

 divided into no less than nine genera ; but by English zoologists they have 

 hitherto been included in only two. Of 

 these, Geomys, as typified by the common 

 pocket-gopher (G. bursarius), is charac- 

 terised by having either one or two 

 grooves on the upper incisors. In the 

 typical species there are two of these 

 grooves, and the same is the case with 

 another form which has been separated 

 as Zygogeomys ; the latter being peculiar 

 in that the jugal, or cheekbone, does not 

 enter into the formation of the upper F[g 59._p OCKET _ G opHER 



border of the zygomatic arch. In all the (Geomys bursar ius). 



other forms, which have received the 



names of Pappogeomys, Orthogeomys, Cratogeomys, Platygeomys, Heterogeomys, 

 and Macrogeomys, there is but a single groove on each incisor ; the groups 

 being distinguished by the conformation of the skull, and the extent 

 to which the cheek teeth are covered with enamel. From the above 

 the species of Thomom.ys are distinguished by the upper incisors being 

 either perfectly smooth, or marked only by a single fine streak on the 

 inner side. 



The kangaroo-rats (Dipodomys), together with the two following genera, 

 are referred by American writers to a different family, and are distinguished 

 by their narrow incisors, moderate or large ears and eyes, and the elongated 

 hind-limbs and tail. In the kangaroo-rats the molar teeth are rootless ; the 

 typical forms having four toes, whereas others, which it has been proposed 

 to separate as Perodipiis, possess five. These small Rodents inhabit open 

 sandy districts, and have habits very similar to those of jerboas. From the 

 foregoing, the genera PerognatJws and Heteromys differ by their rooted 

 molars ; the latter being distinguished by the admixture of flattened spines 

 in the fur, and having species ranging into South America. All the forms are 

 small and mouse-like ; the common D. phillipsi being from South Mexico. 



The jerboas and their allies form a small, widely-distributed family, by no 

 means easy of definition. Usually there is a pair of upper premolars, the 

 incisors are narrow, the molars are divided by transverse 

 folds of enamel into laminae, the vacuity in the skull below Jerboa Tribe, 

 the anterior root of the zygomatic arch is large and rounded, Family 

 and the hinder part of the auditory bulla is very large. The Dipodidce. 

 rat-like Rodents of the genus Sminthus, which range over 

 Eastern and Northern Europe, Central Asia, Kashmir, and Kansu, differ 



