48 



Rats and Mice 



4. NOTES ON STRUCTURE AND CLASSIFICA- 

 TION OF THE MURIDAE, WITH A KEY TO 

 BRITISH SPECIES. 



In these islands everybody is familiar with the general appear- 

 ance of a rat or a mouse ; and this familiarity has often resulted 

 in the names rat " and " mouse " being applied, in consequence 

 of superficial resemblance, to animals which in structure are very 

 different from true rats and mice. Properly these names can only 

 be given to mammals of the order Eodentia ; and then not to all 

 rodents, but only to such species — and there are many hundreds 

 of them — as belong to the family Muridae. We thus learn that 

 in dealing with the rat-like and mouse-like creatures of the world 

 it is not sufficient merely to note their differences or agreements 

 in colour and general external form ; in order to determine their 

 true zoological position we must examine their anatomy in greater 

 or less detail, paying particular attention to the structure of their 

 skulls and teeth — since these organs furnish highly important 

 characters. 



In all Muridae the clavicle or collar-bone is well developed, 

 and the tibia and fibula, the two bones of the lower part of the 

 leg, are, to a great extent, fused together. In most genera the 

 thumb is rudimentary. In the skull (Fig. 2) each frontal bone 

 lacks a postorbital process ; each zygomatic arch is formed 

 chiefly by the zygomatic process of the maxillary and squamosal 

 bones, the jugal being reduced to a mere splint between the two 

 and not articulating with the lachrymal bone in front ; the lower 

 root of the maxillary zygomatic process is flattened into a large 

 more or less vertical plate of bone, which forms the outer wall of 

 the infraorbital canal and gives attachment to that part of the 

 great masseter or cheek-muscle, which is chiefly concerned in 

 pulling the lower jaw forwards in the important work of gnawing. 

 The infraorbital canal is large and shaped like a comma ; its 

 lower and narrower part transmits the sensory facial nerve and 

 vessels, while its upper and wider portion lodges a slip of the 

 masseter muscle. There are never more than sixteen teeth. As 

 in all other rodents (excluding the hares and their allies), four of 

 these teeth are incisors and are placed, one on each side above 

 and below, in the anterior ends of the jaws. These teeth (Fig. 2 D), 

 as the instruments by which the characteristic gnawing function 



