MAMMALIA RODENTIA. 361 



slon only, they are destitute of a clavicle. In most of those 

 which follow, and where a greater development of the limb 

 confers more extensive and more varied powers of motion, 

 applicable to a greater range of objects, this bone is found. 

 In the greater number, however, it is merely in a rudimental 

 state ; that is, developed only to a certain extent, one portion 

 being bony, and the rest cartilaginous ; as if the ossification 

 had been arrested at an early stage. These imperfect clavi- 

 cles are too short to connect the scapula with the sternum ; 

 the rest of the space being eked out by cartilage, and by li- 

 gaments: but, still, they are of great use in affording points 

 of attachment to the muscles of the limb, and giving them the 

 advantage of acting by a rigid lever. The carnivorous tribes, 

 which make considerable use of their fore paws in striking 

 and seizing their prey, have clavicles of this description. 

 Those quadrupeds which have to execute still more complex 

 actions with their fore feet, have perfect clavicles, extending 

 from the shoulder to the chest, and connecting the bones of 

 the anterior extremity with the general frame-work of the 

 skeleton. This is the case in a large proportion of the family 

 of Rodentia, such as the Squirrel^ which employs its paws 

 for holding objects ; and the Beaver,^ which, likewise, makes 

 great use of its feet for moving and arranging the materials 

 of its habitation. Animals that dwell in trees, and require to 

 grasp, with force, the branches, in moving along them, such 

 as the Sloth, have also distinct clavicles. Animals which 

 rake or dig the ground, as the Mole, the Ant-eater, and the 

 Hedge-hog, are all provided with these bones, which, by keep- 

 ing the shoulders at the same constant distance from the 

 trunk, and affording a firm axis for the rotatory motions of 

 the limb, materially assist them in the performance of these 

 actions. 



* The beaver presents a singular modification in the structure of the tail, 

 which is expanded into a flattened oval disk, covered by a skin beset with 

 scales: the whole forming- a mechanical instrument, which maybe compared 

 to a trowel, exceedingly well adapted for the purposes to which it is applied 

 by the animal in constructing its mud habitation. 



Vol. I. 46 



