RODENTIA. 163 



One. RODENTIA. 



Syn. Glires of some. 



Two incisors only in each jaw, large, incurved, and without roots. No 

 canines. Molars separated from the incisors by an interval, usually few 

 in number, varying from two to six on each side, rarely more than four. 

 Feet unguiculate, generally with five toes. 



The so-called incisors are, however, by most anatomists considered the 

 representative of canine teeth, the incisors being obliterated except in 

 hares, which have a pair of incisors behind the upper gnawing teeth.* 



The Rodents or Gnawers are chiefly characterized by the remarkable 

 conformation of their teeth. Their quasi-incisors have a plate of enamel 

 only in front, often coloured yellow or brownish ; behind this the anterior 

 plate of true dentine is also harder than the posterior layer, though not 

 so hard as the enamel. From this structure a sharp edge is effected by 

 the constant attrition, the teeth acquiring a chisel-shape with the slope 

 backwards. The pulp being persistent, these teeth are always growing, 

 and if from any cause the upper tooth is displaced or lost, its antagonist 

 in the lower jaw has been known to grow on, enter the skull, and 

 cause death. The molars have flat crowns, and the enamelled emi- 

 nences, which are always transverse, vary from thin lines to blunt 

 tubercles, according as their diet varies from frugivorous to omnivorous. 

 The condyle of the lower jaw is longitudinal, and slides backwards and 

 forwards, and this motion acting on the peculiar chisel-shaped incisors, 

 serves to reduce the hardest substances by a constant filing or gnawing. 

 The orbits are not separated from the temporal fossae. The inter- 

 maxillaries are enormously developed to hold their large incisors, and 

 the maxillaries are therefore pushed far backwards. The nasal bones 

 are greatly elongated. Some possess clavicles, others have none. The 

 os magnum of the carpus is often divided into two, as in some monkeys. 

 The pelvis resembles that of Carnivora. The fibula is situated behind 

 the tibia, and consolidated with it in the lower portion in many. The 

 os calcis is much developed. 



The stomach is usually simple or in two distinct pouches ; the in- 

 testinal canal is very long, and pretty even in diameter ; and the caecum 

 is generally much developed, and is only absent in one group. The 



* I shall continue to call these teeth the incisors, or quasi-incisors. Blyth has 

 lately named them the rodential tusks. 



M 2 



