88 MAMiVAUA— ORDER V.—RODENTIA. 



ORDER v.— ROCENTIA. 



Mammals that Gnaw. 



Next to the Bats, the order of Mammals most easily defined is the one 

 including the Rodents or Gnawing Mammals, all of which have a very similar 

 and peculiar type of dentition specially adapted for rasping and gnawing 

 hard vegetable substances. The order comprises a large number of families 

 and genera, many of which are exceedingly numerous in species, so that the 

 total number of its members is greater tlian that of any ordinal group in 

 the whole class. As common and well-known examples of the Rodent order 

 may be cited squirrels, dormice, marmots, beavers, rats, voles, porcupii»es, 

 and hares and rabbits, all of which are characterised by possessing a pair of 

 chisel-shaped teeth in the front of each jaw, which are worn by use into a 

 sharp, cutting, transverse edge, and grow continuously throughout the life of 

 their owners. It is with these chisel-like front or incisor teeth that the 

 Rodents perform that gnawing action (so markedly developed in the beavers 

 and porcupines) from which they derive their name ; and it is owing to the 

 circumstance that the front of each tooth is faced with a plate of hard 

 enamel, while the remainder consists of soft ivory, that these beautiful in- 

 struments maintain their cutting-edges. These two pairs of front teeth aro 

 absolutely characteristic of all Rodents; and in by far the greater majority 

 of the order there are no other teeth in this region of the jaw. As if, how- 

 ever, for the purpose of hinting how these animals were originally related to 

 Mammals provided with a fuller set of teeth, the hares and rabbits, together 

 with their near allies the picas or tailless hares, have a minute pair of some- 

 what similar teeth placed immediately behind the large pair in the upper 

 jaw. Being perfectly useless to their owners, this second pair of upper front 

 teeth evidently comes under the category of rudimental or vestigiary struc- 

 tures. Behind the front teeth of all Rodents comes a long gap in each jaw, 

 after which is the series of grinding or cheek teeth, which are never more 

 than six in number, and aro frequently reduced to four, or even three. Con- 

 sequently, no member of the order ever has canine teeth. Were it not that 

 there are two groups of animals with a dentition of a similar type, these 

 peculiarities in the teeth would absolutely distinguish Rodents from all other 

 members of the Mammalian class. Of the groups in question, the one con- 

 tains the wombats of Australia, which are broadly distinguished by the 

 presence of a pouch for the young, while the second group is represented 

 solely by the curious aye-aye of Madagascar, which agrees in its internal 

 anatomy with the lemurs, and is accordingly assigned to that group. "With 

 these exceptions the dentition is absolutely characteristic of the Rodent 

 order ; and as the student is not likely to confound with them either of the 

 creatures named, he may rely on the nature of the teeth in identifying the 

 members of the order. A curious feature in the anatomy of Rodents is that 

 the mouth is divided into two chambers communicating by a narrow orifice ; 

 the first containing the incisors, and the second the cheek teeth, and the 

 hair being continued inwards behind the former. The object of this arrange- 

 ment is evidently to prevent the intrusion of foreign objects into the 



