380 



RODENTIA. 



In the order Rodentia the structure of the 

 thumb differs in different genera ; there is a 

 complete but short thumb in hares, beavers, 

 and jerboas ; an incomplete thumb, consisting 

 of only two phalanges, in squirrels, rats, por- 

 cupines, pacas, and agoutis ; and a thumb, 

 represented by only a single ossicle, in the 

 capybara, the Guinea-pig, the marmot, &c. 



In connection with the fore-arm it may be 

 observed, that the rabbit has only one pronator 

 of the wrist, corresponding to the pronator 

 teres; a circumstance easily accounted for 

 by the very small degree of motion permitted 

 between the bones of the fore-arm ; in most 

 other Rodentia, however, both the pronators 

 are present. 



263. 



Skeleton of the Beaver (Castor Fiber}. 



The terminal phalanges of the fingers are 

 generally very slender, elongated, almost 

 straight, and pointed, except in the capybara, 

 which has its last phalanges of a triangular 

 shape, and enclosed in strong horny hoofs. 



In those Rodentia which have a complete 

 clavicle, the muscles of the shoulder resemble 

 very nearly in their disposition those of the 

 human subject. The humerus resembles 

 that of the Carnivora in its mode of 

 articulation with the fore-arm ; but in those 

 genera that are without clavicles, the articu- 

 lation of the elbow joint resembles more 

 nearly what is met with in herbivorous quad- 

 rupeds, being a simple hinge joint. The hu- 

 merus of the beaver is much expanded at its 

 ulnar extremity, and the deltoid crest is pro- 



In the Rodentia the 

 general form and posi- 

 tion of the pelvis is 

 nearly similar to what 

 exists in the Carni- 

 vora. 



The femur is in the 

 beaver very broad, 

 flattened from before to behind, and exhibits 

 along its outer surface a sharp crest, which 

 represents the linea aspera, and which is pro- 

 longed towards its middle into an apophysis, 

 which has been named the third trochanter. 

 This third trochanter is also met with in other 

 rodents, as, for example, in the musk rat ; in 

 the hares it is placed so high up, that it ap- 

 pears to be a derivation from the great trochan- 



Fig. 204. 



Skeleton of the Flying Squirrel (Pteromys volitans). 



longed inferiorly into a prominent point ; a 

 circumstance which is likewise observable in 

 the rat, the water vole, the ondatra, and in 

 many other genera. 



In the hares, the porcupine, the paca, and 

 the agouti, the humerus near the elbow joint 

 is completely perforated. 



ter ; in other genera, as in 

 the water vole, the rats, the 

 squirrels, and the marmots, 

 it is a simple crest, or ex- 

 ternal iinea aspera ; in all 

 the Rodentia the great tro- 

 chanter is very prominent, 



and the neck of 



