446 RODENTIA 



In the cursorial and leaping species the lumbar transverse processes 

 are generally very long, and in the Hares there are large com- 

 pressed hypapophyses. The caudal vertebra? exhibit great variety 

 in structure, being in a rudimentary condition in the Guinea-Pig, 

 while in the Jumping Hares and prehensile-tailed Porcupines they 

 are of very large dimensions. The scapula is usually narrow, with 

 a long acromion ; the clavicles may be altogether absent or imper- 

 fect, as in the Porcupines, Cavies, and Hares, but in most species 

 they are well developed. In all existing forms the humerus has 

 no entepicondylar foramen, and the radius and ulna are distinct. 

 In most species the manus has five digits, with phalanges normally 

 developed ; the pollex being rarely rudimentary or absent. The 

 pelvis has well-developed ischia and pubes, meeting in a long, and 

 usually bony, symphysis. The femur varies considerably in form, 

 but generally has a well-defined third trochanter ; in the Sciurine 

 and Hystricine Rodents the tibia and fibula are distinct, but in the 

 Rats and other Murines, and in the Hares, these bones are united, 

 often high up ; the pes is much more variable than the manus, the 

 digits varying in number from five, as in the Squirrels and Rats, to 

 four, as in the Hares, or even three, as in the Capybara, Viscacha, 

 and Agouti ; in the Dipodidce the metatarsals are greatly elongated, 

 and in some of the species, as in the Jerboas, they are ankylosed 

 together. 



The mouth is divided into two cavities communicating by a 

 constricted orifice, an anterior one containing the large incisors, and 

 a posterior one in which the molars are placed ; the hairy integu- 

 ment of the face being continued inwards behind the incisors. This 

 peculiar arrangement evidently prevents substances not intended 

 for food getting into the mouth, as when the animal is engaged in 

 gnawing through an obstacle. In the Hares and Pacas the inside 

 of the cheeks is hairy, and in some species, as in the Pouched Rats 

 and Hamsters, there are large internal cheek-pouches lined with 

 the hairy integument, which open near the angles of the mouth 

 and extend backwards behind the ears. In the New World 

 Pouched Rats (Geomyidcv) the pouches open externally on the 

 cheeks. The tongue presents little variability in length, being 

 always short and compressed, with an obtuse apex never protruded 

 beyond the incisors. In most species there are three circumvallate 

 papillae at the base ; and the apical portion is generally covered 

 with small filiform papillae, some of which in the Porcupines 

 (Hystrix} become greatly enlarged, forming toothed spines. The 

 stomach varies in form from the simple oval sac of the Squirrel to 

 the complex ruminant-like organ of the Lemming. In the Water- 

 Vole (Arvicola amphibius) and the Agouti (Dasypi-oda. aguti) it is 

 strongly constricted between the oesophagus and pylorus. In the 

 common Dormouse the oesophagus immediately before entering the 



