104 RODENTIA order vi 



this P, and in the geologically older rodents it is more complicated, and 

 functions for a greater length of time, than in the more recent forms, where 

 the replacement often occurs in the foetus, as in the case of Caviinae. In 

 the embryo of Sciurus AdlofF has seen traces of the formation of three incisors 

 and one canine. 



Rodents at the present day exceed all other mammalian orders in number 

 of genera and species. More than 900 living species have been counted, 

 of which a large portion inhabit South America. North America and 

 Eurasia have many genera in common. In Africa, in addition to a number 

 of peculiar types, are found representatives of European and Asiatic families 

 and genera. 



In spite of their small size and delicate structure the number of 

 fossil remains of rodents is consideri3,ble. In Europe quite a number 

 of species from the Upper Eocene and Oligocene are known, especially 

 from the Phosphorites of Quercy and from the Miocene deposits in the 

 Department of Allier, those of Mayence and Ulm, as well as of Sansan 

 (Dept. Gers), La Grive St. Alban (Dept. Isere), of Steinheim" and 

 from the Ries near Nordlingen. In North America the rodents are 

 less abundant, and are found chiefly in the Bridger Eocene, in the White 

 River Oligocene, and in the John Day and Loup Fork Miocene. South 

 America, on the other hand, possesses an abundant rodent fauna, found in 

 the Santa Cruz formation of Patagonia and in the early Tertiary and Pleisto- 

 cene of Argentina. Rodent remains have been discovered also in several 

 Pleistocene cave deposits of Europe, North and South America. 



The classification of the rodents offers many difl&culties, for though the 

 two suborders Duplicidentata and Simplicidentata are well defined, yet the 

 arrangement of certain forms of the latter group under the three principal 

 subdivisions employed up to the present time, namely, Sciuromorpha, 

 Myomorpha and Hystricomorpha, is very difficult. The formation of a fourth 

 subdivision, Profrogomorpha, is only a makeshift, which would serve to confuse 

 the relationship between many fossil forms and their living representatives, 

 as would also the proposed plan of Tullberg, to divide all the forms into the 

 two tribes Sciurognathi and Hysfricognathi. The most feasible plan of classifica- 

 tion seems to be that of AYeber, which has been adopted here with several 

 additions. 



Suborder A. SIMPLICIDENTATA. 



There is always only \ long incisor. Premolars f, more frequently \, often ■§-. 

 Enamel on the incisor is almost always yellow and restricted to the front surface of 

 the tooth. The dentition of the upper jaw is more compact than that of the lower. 

 The articulation for the lower jaw is narrow. Scaphoid and lunar are mostly fused. 

 Fibula not articulating with the calcaneum. 



Family 1. Aplodontoidea Gill. 



\ P. |- il/. Cranial roof flat. Masseter not extending through or beyond 

 the small infraorbital foramen. A^o postorbital process. Tibia and fibula not 

 fused. 



