RODENTIA. 421 



idee, including hares, rabbits, and Picas or tailless hares of 

 Siberia, retaining a second minute incisor behind each of the 

 larger ones in the upper 

 jaw. 



Some parts of the 

 skeleton, and more espe- 

 cially the dentition of the 

 rodent order, are highly 

 characteristic — the form 

 of the articular surface 

 for the lower jaw, which 

 is a longitudinal groove, &• • 



Skull and teeth of a Porcupine. 



— the molars, especially 



of the phytiphagous kinds, crossed by enamel plates more or 

 less transverse — these, with the long, curved, chisel-shaped in- 

 cisors, two in each jaw, suffice to determine the ordinal rela- 

 tions of the fossil. The incisors alone would not be always 

 so safe a guide, for the rodent modification of these teeth is 

 repeated in the marsupial wombat and the lemurine aye-aye. 



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 forma- 

 tions. 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 Rodents 

 allied to the dormouse (Myoxus) and squirrel (Sciurus) in the 

 eocene building-stone of the Montmartre quarries near Paris. 

 The lacustrine marls of the middle (miocene) tertiary period 

 have yielded evidences of not fewer than eleven genera of 

 Rodentia distinct from any now known to exist. The deposits 

 at Eppelsheim, near Darmstadt, of the same miocene age, have 

 given evidence of Rodents akin to the marmot and the beaver. 



