448 RODENTIA 



the Beaver, or again, into pouches opening near the anus, as in the 

 Hare, Agouti, and Jerboa. 



The integument is generally thin, and the panniculus carnosus 

 (the sheet of muscle underlying the skin) rarely much developed. 

 The fur varies exceedingly in character. Thus it may be very 

 fine and soft, as in the Chinchillas and Hares, in others more 

 or less replaced by spines on the upper surface, as in the Spiny- 

 Eats and Porcupines ; in several genera, as in Xerus, Acanthomys, 

 Platacanthomys, Echinothrix, Loncheres, and Echinomys, the spines are 

 flattened. In the muscular structures the chief peculiarities are 

 noticeable in the comparatively small size of the temporal muscles, 

 and in the great double masseters (Fig. 194), which are the prin- 

 cipal agents in gnawing ; the digastrics also are remarkable for their 

 well-defined central tendon, and in many species their anterior bellies 

 are united between the mandibular rami ; the cleidomastoid generally 

 arises from the basioccipital, and the pectoralis major is connected 

 with the latissimus dorsi ; in the Porcupines and Hares the tendons 

 of the flexor digitorum longus and flexor hallucis longus are con- 

 nected in the foot, while in the Pats and Scpiirrels they are separate, 

 and the flexor digitorum longus is generally inserted into the 

 metatarsal of the hallux. 1 



Rodents are tolerably well represented in a fossil condition from 

 the period of the Upper Eocene ; while if Decticadajns, of the Lower 

 Eocene of Rheims, is rightly referred to it the order dates from the 

 oldest Tertiaiy. All the fossil forms at present known are, however, 

 essentially true Rodents, and afford no clue as to the relations of 

 the order with other mammals. The remote affinities of the 

 Rodents to the Proboscidea, as well as their more marked resem- 

 blances to Typotkerium, have been already mentioned. "Whether 

 there is a real genetic affinity (as Professor Cope suggests) with the 

 Tillodontia cannot be decided with the evidence at present available. 



Suborder Sdiplicidentata. 



Only one pair of upper incisors, having their enamel confined to 

 their front surfaces. Incisive foramina moderate and distinct ; 

 fibula not articulating with the calcaneum. Testes abdominal, and 

 descending periodically only into a temporary sessile scrotum. 



Section Sciuromorpha. 



Zygomatic arch slender, chiefly formed by the jugal, which is 

 not supported by a long maxillary process extending backwards 

 beneath it ; postorbital processes of frontal present or absent ; 



1 See G. E. Dobson, Journ. Anat. Phys. vol. xvii. 



