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- 
Rats and Porcupines; in several genera, as in Xerus, Acunthomys, 
Platacanthomys, Echinothriz, 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 Poreupines and Hares the tendons 
of the flexor digitorum longus and flexor hallucis longus are con- 
nected in the foot, while in the Rats and Squirrels they are separate, 
and the flexor digitorum longus is generally inserted into the 
metatarsal of the hallux.t 
Rodents are tolerably well represented in a fossil condition from 
the period of the Upper Eocene, while if Decticadapis, of the Lower 
Eocene of Rheims, is rightly referred to it the order dates from the 
oldest Tertiary. 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 Typotherium, 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 SIMPLICIDENTATA. 
Only one pair of upper incisors, having their enamel confined to 
their front surfaces. Incisive foramina moderate and distinct : 
fibula not articulating with the caleaneum. Testes abdominal, and 
descending periodically only into a temporary sessile scrotum. 
Section SCIUROMORPHA. 
Zyzomatie 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. -trat. Phys. vol. xvii. 
