RODENTIA. 



381 



the thigh bone considerably narrower than its 

 head. 



its posterior aspect there is likewise a promi- 

 nent crest. It results from this structure, 



Fig. 265. 



Skeleton of the Paca (Ccelogenys Paca). 



The Rodentia have the fibula situated quite 

 behind the tibia ; in rats, voles, jerboas, the 

 beaver, the helamys, and the rabbit, it becomes 

 consolidated with the tibia towards the lower 

 third of its length, a wide triangular space 

 being left between the two bones at the 

 upper part of the leg ; the anterior crista of 

 the tibia in all the above genera is remarkably 

 prominent, as is the internal edge ; and upon 



that, viewed from behind, the tibia exhibits, in 

 the upper half of its length, two deep fossae 

 for the attachment of the tibialis posticus 

 and the flexor longus pollicis. This struc- 

 ture is more particularly remarkable in the 

 ondatra. 



In the beaver the fibula gives off' from its 

 upper extremity a strong recurrent apophysis, 

 which is directed slightly outwards. In some 

 genera the fibula is excessively slender, and 

 does not reach so low down as to become 

 connected with the lower extremity of the 

 tibia. 



In those Rodents which have the fibula con- 

 solidated with the tibia towards its tarsal ex- 

 tremity, the tarsus seems to be articulated 

 with the latter bone only ; but if very young 



Skeleton of the Jerboa (Dipus hersipes). 

 (Altered from Pander and D' Alton.) 



individuals are examined carefully, it is per- 

 ceptible that the external malleolus is formed 

 by the fibula. 



In Rodentia the os colds is very much 

 elorg ted posteriorly. 



In such genera as have five complete toes the 

 following circumstances may be remarked : 

 In the beaver the os scaphoides is divided 

 into two portions, one placed in front of the 

 astragalus, which supports the second and 

 third cuneiform bones, and one placed inter- 

 nal to the astragalus, to which is attached the 

 cuneiform bone that supports the great toe, 



