642 BIOLOGT. 



at another too many teeth ; molars uniform, with per- 

 fectly irregular incisors and canines Sloths, Marsupials. 

 Others, lastly, have a tolerably regular set of teeth, 

 presenting quadriacuminate molars, mostly small canines, 

 and rodent-like incisors Shrews and Bats. 



Order 1. Geddrm-, Eyer, Schleimthier-Haarthiere 

 Rodentia. 



3565. Rodent teeth, without canines. 



The dental formula of the gnawing Rats and Mice is 

 so varied, and so devoid of influence upon their bodily 

 structure and mode of life, that all attempts to arrange 

 these animals in accordance with it have failed. 



3566. The lower position of the Rodentia admits of 

 being easily proved by taking to our aid the meaning of 

 the dental system. The splanchnic or visceral teeth 

 obviously rank lower than the membral teeth. Now, it 

 so happens that the anterior teeth are in the Rodentia 

 the principal organ, not only of the dental formula, but of 

 the entire animal. Without gnawing-teeth the Mice 

 could not maintain their existence, much less their cha- 

 racter. They thus depend wholly and solely upon the 

 visceral teeth, and are consequently the lowest Thri- 

 cozoa. 



In addition to this comes their small size, which is by 

 no means a character to be despised, and one to which all 

 naturalists pay regard, even while they keep it, upon the 

 other hand, in the background ; or else they must place 

 the Elephant below the Field-mice. 



3567. I divide them, as well according to structure as 

 habit, into three groups. The one has the front and 

 hind feet of equal length, with blunt claws for digging. 



The other has similar feet, with sharp claws for 

 climbing. 



Lastly, the third has the hind feet longer than the fore 

 for leaping. 



Fam. 1. Infusorial Thricozoa WuJdmduse. 



Eyes and auditory conchae feebly developed, feet for 

 scraping, tail lax ; live always under the ground, and eat 



