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 vertebre 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 Dipodide 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 (Geomyidw) 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 
papille at the base; and the apical portion is generally covered 
with small filiform papille, some of which in the Porcupines 
(Hystriz) 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 umphibius) and the Agouti (Dusyprocta agutt) it is 
strongly constricted between the cesophagus and pylorus. In the 
common Dormouse the esophagus immediately before entering the 
