84 OSTEOLOGICAL GALLERY. 



starvation, or by curving right over and entering the back of the 

 head. An example of the results of such an accident is shown in 

 the skeleton of Gould's Hapalotis (Hapalotis gouldii) exhibited in 

 Case 8, Div. B. The clavicles are very varied in their develop- 

 ment, being, as is usual throughout the Mammalia, complete from 

 end to end in those forms, such as the Squirrels, in which the 

 anterior limbs are used for grasping or climbing ; while they are 

 incomplete 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 Simplicidentata (see fig. 44), 

 while those that have a second smaller pair behind the large front 

 ones are called Duplicidentata (fig. 45). 



[Case 8.] 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 (Anomalurus) , the true Squirrels (Sciurus), 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 

 up into dams for their dwelling-places. 



The Myomorpha, 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 are exhibited skeletons of Dormice (Myoxus), 

 common Rats and Mice (Mus), Voles (Microtus), Mole- Rats 

 (Spalax), Pouched Rats (Geomys), Jerboas (Dipus), and numerous 

 others. 



The Hystricomorpha, or Porcupine tribe (Divs. C E), have 



