xxv. 3 RODENT CHARACTERS 657 



the medial passes out of the orbit through the much-enlarged infra- 

 orbital canal. In the porcupines and their allies (Hystricomorpha) 

 the lateral portion remains simple and the medial proceeds to a large 

 insertion on the face below (but not through) the infra-orbital canal. 

 The lower jaw often carries a large flange, for the attachment of the 

 masseter muscle. The pterygoid muscles and their attachments are also 

 often large, but the temporal muscle is usually small. 



Apart from their gnawing mechanism the rodents remain rather 

 unspecialized mammals. The gait is plantigrade, the fore-limbs often 

 shorter than the hind and used for handling the food. In some this 

 tendency is carried to the extent of producing a hopping, bipedal gait. 

 The gut shows a large coecum and food is passed twice through the 

 gut (p. 662). A division of the stomach is found only in mice, which 

 have a horny cardiac region. Rodents are macrosmatic, with large 

 olfactory bulbs and relatively small, smooth neopallium. The eyes are 

 often well developed, especially in arboreal rodents and those living 

 in open country or steppe. Hearing is often good, and in some desert 

 species the tympanic bulla is greatly dilated, perhaps to detect sounds 

 made by the widely separated individuals. Many rodents have well- 

 developed social habits, with olfactory and visual, as well as auditory 

 and tactile signalling. 



Rodents are mostly polyoestrous, often breeding throughout the 

 year, at least in captivity. Numerous young are produced and are often 

 cared for in a nest. The uterus is usually double and the placentation 

 usually of the discoidal and haemochorial type. 



No thoroughly satisfactory scheme for grouping the various types 

 of rodent has been devised ; the best that we can do is to keep to the 

 classical division of the order into three suborders, Sciuromorpha, 

 Myomorpha, and Histricomorpha. The first includes besides the more 

 primitive surviving forms, Aplodontia, the mountain beaver of North 

 America, also the most ancient fossil rodents (*Ischyromyidae) and 

 some families of uncertain affinities. The characteristics of the sub- 

 order are that there are two upper premolars and one lower and a 

 masseter muscle that does not pass through the infra-orbital canal. 

 The suborder includes the squirrels (Sciuridae) found in all major 

 regions except Australasia. They are diurnal, with large eyes and often 

 bright colouring. The flying squirrels, Petaurista, glide for long dis- 

 tances by means of the patagium, whose muscles enable them to 

 change direction in the air (Fig. 424). 



The marmots (Marmota, Fig. 425), ground squirrels (Citellus), and 

 prairie dogs {Cynomys) are burrowers, with elaborate underground 



