MAMMALIA RQDENTIA. 523 



less modifications of structure that have been 

 executed in conformity with those plans, I am 

 able particularly to notice only such as are most 

 remarkable. 



8. Rodentia. 



As the tribes of mammalia we have hitherto 

 examined employ the anterior extremities for 

 the purposes of progression only, they are desti- 

 tute of a clavicle. In most of those which follow, 

 and where a greater developement 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 clavicles are too short to connect 

 the scapula with the sternum ; the rest of the 

 space being eked out by cartilage, and by liga- 

 ments : 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 



