THE RODENTS 189 



independent histories from at least the Basal Eocene, a great 

 length of time which has permitted the appearance of many 

 convergent characters. 



In the Basal Eocene Ave find the Rodents had already attained 

 obviously ordinal characters as in the case of the Bats, the Eden- 

 tates, the Cetacea and all the Ungulate orders except the Artio- 

 dactyla. Therefore we must infer that the point of divergence 

 of the Rodents from the general mammalian ancestors must lie 

 well back in Cretaceous times. It is this order which, above all 

 among Placentals, shows most the influence of the incisor modi- 

 fication in determining the characters of the dentition. We be- 

 lieve the ancestral Rodents were semi-arboreal Mammals with 

 rather large procumbent incisors as in some modern Insectivores, 

 canines and anterior premolars reduced and bunodont molars 

 adapted for an omnivorous diet. The Eocene genus Paramys 

 displays characters of this nature. 



Starting as we have usually done with a less specialized form 

 Ave shall describe the dentition of Arctomys first, 



Arctomys monax, the Woodchuek or Ground Hog (Fig. 67), 

 is a Marmot of burrowing habit and lives upon grass and 

 clover. Although fossorial it can and does climb trees both for 

 pleasure and for protection. The Rodent features of the skull 

 are recognized at once. Its molar teeth do not greatly differ 

 from the typical mammalian form, the hinder ends of the upper 

 tooth rows converge but slightly and the palate ends a short 

 distance back of the last molar, three features previously 

 noted as indicating a comparatively slight degree of speciali- 

 zation. The tooth formula is: 



llc§,pf*f. 'total 22. 



The specimen photographed is not a completely adult skull: 

 the first lower and the anterior upper cheek teeth belong to 

 the milk dentition and are replaced in later life by stouter 

 and more molariform premolars. The incisors are enamel- 



