630 RODENTIA. 



middle of the zygomatic arch, but is occasionally prolonged 

 back to the glenoid cavity. The orbit is not closed behind 

 and the frontal is without postorbital processes except in a 

 few genera. The lacrymal foramen is intraorbital, and the 

 infraorbital foramen is often very large, sometimes as large 

 as the or it (Hystricidae, etc.), transmitting part of the 

 masse ter muscle. 



The nasals extend far forward and the premaxillae are 

 large. There is usually an interparietal bone. The palate 

 is narrow and the anterior palatal foramina extensive. The 

 angular part of the mandible is considerable and the coronoid 

 process low. 



The dorsolumbar vertebrae are usually nineteen. 



Clavicles are usually well developed but they may be feeble 



or absent. The scapula is narrow, and has an acromion 



usually with a metacromion. The humerus is without an 



entepicondylar foramen, and the femur often has a third 



^, trochanter. The radius and 



ulna are distinct and some- 

 times capable of rotation, but 

 the tibia and fibula are often 

 united. The carpus has a cen- 

 trale and the scaphoid and 

 lunar are generally fused. 



^Tfrom cilS!^ °* *^'^""* "^^^ T^®^® ^^® usually five digits 



in the manus but the pollex 

 may be reduced or absent. In the pes the digits vary from 

 five to three and in the Dipodidae the metatarsals are much 

 elongated and may be ankylosed. 



The brain is small and the cerebellum is left uncovered by 

 the cerebrum, the surface of which is usually but slightly 

 convoluted. In the larger forms the convolutions are better 

 developed and in the smaller they are absent, the surface 

 being quite smooth. 



The hairy integument of the face is often prolonged into 

 the mouth behind the incisors, and there is thus a kind of 

 antechamber to the mouth in which gnawed matter not 

 intended for food may be intercepted. The hairy lining 

 may extend even into the cheek pouches if these are present. 

 In the Geomyidae the cheek pouches open externally on the 



