ORDER RODENTIA. 99 



the object intended by the means employed, but when we do 

 so in fact, or fancy we have fully done so, we may sometimes 

 find the general means not dispensed with, in cases where 

 the particular ends do not seem to be contemplated. 



As those species which have more than three cheek- 

 teeth, do not, in fact, change the posterior three, we may 

 probably conclude that those which have only three in all 

 never change either of them ; accordingly, the Baron could 

 never detect any mutation of cheek-teeth in these latter 

 species, as the Rats, fyc. 



The Rodentia are totally destitute of canine teeth, and 

 the space between the incisors and the cheek-teeth is void. 



The lower jaw is articulated by a longitudinal condyle, 

 so as to confine its movement to a horizontal motion forward 

 and backward. 



It seems almost needless to remark, that this dentary for- 

 mation is in strict conformity with the habit proper to these 

 animals, of dividing hard substances with the incisors, and 

 triturating the pieces with the cheek-teeth. 



The head of the Rodentia is flat above, the frontal bone 

 is divided by a medial suture, the parietal is single in some, 

 as the Marmots, Hare, Squirrels, fyc. ; and double in 

 others, as the Rabbit, the Mouse, and the Dormouse, 8fC. ; 

 the opening of the nasal fossae is vertical at the end of the 

 muzzle, and is heart-shaped, with the point downward ; the 

 orbits are nearly round, and communicate with the temporal 

 fossae, which are very large. Some genera have perfect 

 clavicles, and others have them only in a rudimentary state; 

 they have the two bones of the fore-arm, but these are fre- 

 quently attached to each other ; the toes vary, and will be 

 found in the generic characters ; the posterior extremities 

 are always larger than the anterior, and are sometimes 

 extremely elongated. 



The brain is small, and has but few circumvolutions ; 

 the eyes in some species are very large, in others, as the Mole 



