84 
OSTEOLOGICAL GALLERY. 
starvation, or by curving right over and entering the hack of tlie 
head. An example of the results of such an accident is shown in 
the skeleton of Gould^s Ilapalotis {Hapalofis youldii) exhibited in 
Case 8, Div. B. The clavicles are very varied in their develop- 
ment, being, as is usual throughout the iMammalia, complete from 
end to end in those forms, sueh as the Squirrels, in whieh the 
anterior limbs are used for grasping or climbing; while they are 
incom})lete or absent in those which live a simple terrestrial life, 
and use their anterior limbs for 'walking or digging only. 
The mandible is articulated to the skull by a longitudinal hinge, 
which gives a large amount of mobility and freedom to the jaw, at 
a corresponding sacrifice of strength and rigidity. 
As already mentioned (p. 31), Rodents are divided into two 
Suborders, according to the number of their upper incisors, those 
with only a single pair being the SimjjHcide?itata (see fig. 44), 
while those that have a second smaller pair behind the large front 
ones are called Duplicidentata (fig. 45). 
[Case 8.J Of the Simplicidentata, the Squirrel section (Div. B, above) are 
distinguished by always having at least one premolar, by having a 
flattened, not twisted, lower jaw, small palatine foramina, and by 
their two shin-bones, the tibia and fibula, always remaining separate 
from each other during life. They generally have well-marked 
postorbital processes. To this group belong the Scaly-tailed Flying 
Squirrels [Anomaliirus) , the true Squirrels [Sciiirus), the Beavers 
[Castor), &c. The Beaver’s incisors are, perhaps, the finest exam- 
ples of gnawing-teeth, being the instruments with which that 
animal is able to cut down good-sized forest-trees, to build them 
u}) into dams for their dwelling-places. 
The Myomorpho, or Rat tribe (Div. B below), have a variable 
number of premolars (0-3), a flattened lower jaw, no postorbital 
processes, very long palatine foramina, perfect clavicles, and their 
tibia and fibula are always joined to each other about halfway down 
(see fig. 43) . Of the large number of genera and species belonging 
to this tribe, there arc exhibited skeletons of Dormice [Myoxus), 
common Rats and j\lice [Mus], Voles [Microtus), Mole-Rats 
[Spalux], Pouched Rats (Gco?/??/s), Jerboas [Dipus), and numerous 
others. 
The Hystricomorpha, or Porcupine tribe (Divs. C-E), have 
