CHAPTER XI 



RODENTI^ (continued). SQUIRRELS, BEAVERS, 

 DORMICE, AND RATS 



SUB-ORDER: SIMPLICIDENTATA. RODENTS WITH ONLY ONE 

 PAIR OF INCISOR TEETH IN BOTH JAWS 



THIS group includes by far the largest number of Rodent 

 species, perhaps in proportion to Duplicidentata something 

 like three hundred to twenty. Its most archaic representa- 

 tives are the squirrels, which, except for not possessing more 

 than a single pair of incisors on both jaws, are in some respects 

 more generalised than the hares and rabbits. Although mainly 

 of small size, and offering a remarkable external resemblance one 

 to the other in the smaller forms (so that an ignorant observer 

 might class all the Simplicidentata in a single family), there are 

 nevertheless remarkable differences amongst these Rodents in the 

 number and construction of their molar teeth, in the possession 

 or loss of collar bones, and the grooving or non-grooving of the 

 incisors, as also in the existence of the cascum. They are divided 

 into three principal groups, which are termed respectively squirrel- 

 like (Sciuromorpha\ rat-like (Myomorpha\ and porcupine-like 

 (Hystricomorpha). The last-named is not represented at all in the 

 British fauna past or present, though early and existing types 

 of porcupines once inhabited France and Germany. The other 

 two sections are divided into a great number of distinct families, 

 of which four are represented in the types of recent or existing 



British Mammals. 



224 



