RODENTIA. 377 



foramen ; the radius and ulna are separate and stout at their lower end; 

 the man us is plantigrade and comprises five complete digits with sharp, 

 laterally-compressed claws. The femur bears a third trochanter ; the 

 fibula is comparatively slender and somewhat arched ; the astragalus is 

 depressed, only slightly grooved above ; the pes resembles the manus. 

 The typical species Tillotherium fodiens (fig. 213), from the Middle Eocene 

 (Bridger Formation) of Wyoming, is known by the greater part of the 

 skeleton. The skull measures O32rn. in length, and the animal must 

 have been a little more than half as large as a tapir. 



Sub-Order 2. Duplicidentata. 



A few of the surviving rodents still retain more than one 

 pair of incisors in the upper jaw ; and these teeth are not com- 

 pletely chisel-shaped (or scalpriform), their enamel not being 

 restricted to a band on the front face but also extending con- 

 siderably round the sides. In the newly-born young of these 

 animals there are three pairs of upper incisors ; but the outer 

 pair soon disappears, while the second pair remains relatively 

 small and is placed directly behind the middle functional pair. 

 The ordinary single enlarged pair of incisors in the lower jaw 

 opposes these teeth. The insertion of the masseter muscle on 

 the mandible has not extended further forwards than the posi- 

 tion of the hindermost molar. A primitive feature is also 

 retained in the hind limb, the fibula still articulating with the 

 calcaneum. 



The rodents thus characterized are not only in a minority in 

 the existing fauna, but have always been few in number com- 

 pared with those retaining only one pair of upper incisors (the 

 Simplicidentata). No extinct families are known, all the fossil 

 genera and species being closely related to the existing hares 

 and rabbits (Leporidae) or to the tailless hares (Lagomyidae). 

 Palceolagus represents the first family in the Lower Miocene 

 (White River Formation) of North America, and seems to have 

 survived until the Pliocene. True hares and rabbits (Lepus) 

 also occur in the Pliocene of Europe, Asia, and North America, 

 and they reached South America at the dawn of the Pleistocene 

 period. The tailless hares are known only from the Miocene 

 and Pliocene of Europe, and seem to have reached North 

 America at the close of the Pliocene period. The surviving 



