SUBORDER A 



SIMPLICIDENTATA 



107 



Castor Linn. Beaver (Fig. 133). Teeth very hypsodont. Eecent in 

 Europe and North America. Fossil in Europe in the Pliocene and Pleistocene, 

 and in North America in the Pleistocene. 



Trogonthermm Fischer. Lower Pleistocene of 

 Europe. 



Castoroides Forster. About the size of a bear. 

 Pleistocene of North America. 



Eucastor Leidy (Sigmogomphius Merriam). 

 Miocene of North America. Bipoides Jager, in 

 the Pliocene of Europe, China, and in the Miocene 

 of North America. These are distinguished by 

 a small number of enamel folds. 



Fig. 132. 



Steneojiber eseri H. v. Meyer ( = Si. 

 viciacensis Gervais). Lower Miocene 

 of St. Gerand-le-Pny, Allier. Lower 

 aspect of skull. 3/^. (After Filhol.) 



Fig. 133. 



Longitudinal vertical section through the skull of a beaver (Castor 

 fiber Linn.) to show the roots of the incisor and the molars. (After 

 Flower.) 



Eutypomys Matthew has two upper P. Cheek teeth with many small 

 enamel fossettes. Oligocene (White River beds). 



Subfamily 2. Mylagaulinae. 



Skull very broad with postorbital process. Cheek teeth high, with many enamd 

 fossettes elongated mostly parallel to the jaw. Extremities stout, adapted for digging. 

 Tibia and fibula free. In the Miocene and Lower Pliocene of North America. 

 According to Riggs these are the descendants of Meniscomys. 



Mylagaidus Cope ; Mesogaulus Riggs ; Ceratogaulus Matthew (with bony 

 horn-core on the nasals) ; Epigaidus Gidley. 



Family 4. Geomyoidea. 



\ P. % M. Skull usually with small infraorbital foramen, without postorbital 

 process, with large ossified bulla. Cheek teeth mostly hypsodont with one fold through 

 the centre of the tooth. Tibia fused with fibula. Only in North America. 



Subfamily 1. Protoptychinae. 

 Infraor'bital canal large. Teeth with roots. 

 Protoptychus Scott. Upper Eocene (Uinta beds) ; North America. 



