RODENTIA 



385 



changes during which the larger species perished. It is pro- 

 bable that the horse and ass are descendants of species of 

 pleistocene antiquity. At the pliocene period there existed a 

 species similar in size to a zebra. There is no certain character 

 by which the present wild boar can be distinguished specifically 

 from the Sus, which was contemporary with the mammoth. 



Order Rod en ti a. 



This order includes an extensive series of small Mam- 

 mals in which a single pair of large, curved, ever-growing- 

 incisors in each jaw is associated with many other peculia- 

 rities of structure. These 

 incisors (fig. ] 83, i), sepa- 

 rated by a wide interval 

 from a short series of 

 molars, characterize the 

 whole order of Eodents ; 

 the single exceptional 

 family, Leporidw, includ- 

 ing hares, rabbits, and Fig. 133. 

 Picas or tailless hares of sku11 and teeth of a Porcupine. 

 Siberia, retaining a second minute incisor behind each of the 

 larger ones in the upper jaw. 



The small size of the great majority of the species of this 

 order leads to the neglect or the oversight of their fossil 

 remains by the labourers in quarries and other deposits of 

 stone, to whom the palaeontologist is usually indebted for 

 his first acquaintance with characteristic fossils of such 

 formations. No evidence has yet been obtained of any 

 unequivocal remains of a rodent animal in strata more 

 ancient than the eocene tertiary deposits. Cuvier detected 

 remains of Eodents allied to the dormouse (Myoxiis) and 

 squirrel (Sciurus) in the eocene building-stone of the Mont- 

 martre quarries near Paris. The lacustrine marls of the middle 



2 c 



