xxv. 3 TEETH OF RODENTS 655 



gives them chisel edges. If one incisor is lost the other continues to 

 grow round in a spiral, until it enters the skull. 



The remaining incisors, canines, and anterior premolars are missing, 

 leaving a large diastema in front of the cheek teeth. Folds of skin can 

 be inserted into this gap to close off the front part of the mouth, so 

 that material bitten off during gnawing is not necessarily swallowed. 

 A distinct anterior chamber of the mouth is thus formed, and may be 

 prolonged into deep pockets in which food is stored for transport to 



Fig. 422. Diagrams of lower jaws of Sciurus. 



A, at rest; B, opened for seizing; C, closed for gnawing or to prise open a nut. 



mlp. deep part of lateral masseter; mis. superficial part of the same; tm. trans- 



versus mandibulae. (From Weber after Krumbach.) 



the hoards that are collected by many species. The teeth are not used 

 only for obtaining food ; rats will gnaw their way through a lead pipe. 

 The premolars are reduced to two above and one below in the more 

 primitive squirrels; even fewer are present in other rodents. The 

 molars and premolars are usually alike in pattern and show modifica- 

 tions similar to those found in the grinders of other herbivorous 

 mammals. The cusps of the original eutherian molar can still be 

 recognized in the squirrels, but they are arranged in transverse rows, 

 paracone and protocone in front, metacone and hypocone behind. The 

 cusps become joined in pairs to form ridges, giving a bilophodont 

 grinding tooth. In most of the rodents further ridges are then added 

 in front, behind, and between the original ones, and these are also 

 joined by cross-ridges, giving a multi-lophodont molar. Similar 

 changes occur in the lower teeth. The teeth also become very high- 

 crowned or 'hypsodont' and the enamel, dentine, and bone ('cement') 

 wear at differing rates. The roots remain wide open and the teeth 

 grow continually. All of these features are like those arrived at in 

 ungulates by a convergent process of evolution. A further feature of 



