812 THE WHITE KIVEE FAUNA. 



RODENTIA. 



Members of this order were very abundant during the White River and 

 Truckee epochs in North America. They are referable to thirty-one species 

 and eight genera. Of these genera three still exist in the regions where 

 their fossil remains are found. These are Sciurus, Hesperomys, and Lepus. 

 All of them occur in the Truckee beds, while the first named only has been 

 found in the White River formation. All of the species belong to the three 

 great divisions of tlie order which now inhabit North America, while the 

 fourth, the Hystricomorpha, which is very sparingly represented on the con- 

 tinent, has not yet been detected in the formations in question. It appears 

 in a single species of porcupine in the Loup Fork beds. 



The four primary divisions of the order Bodentia are thus defined, prin- 

 cipally after Brandt and Alston : 



I. Incisor teeth |. Fibula not articulating with the superior condj-le of the calcaneum. 



Xo iutertrochlear crest of humerus. 



1. Mandible with the angular portion springing from the outer side of the bony 



covering of the lower incisor. Fibula distinct from tibia. " Malar bone not 

 supported below by a continuation of the maxillary zygouuxtic process." An 

 inter])terygoid fissure Hyxlricomorpha. 



2. Mandible with the angle in the plane of or springing from the inferior edge of 



the covering of the alveolus of the inferior incisor, more or less rounded ; coro- 

 noid process high, falcate. Fibula distinct from tibia. No interpterygoid 

 fissure Sciuromorpha. 



3. Mandible with the angular portion springing from the inferior edge of the sheath 



of the inferior Incisor (except Bathyergiiue). Fibula coossified with the tibia. 

 Malar short, usually supported on a maxillary process. No interpterygoid 

 fissure (except in Baihyergincc) Myomorpha. 



II. Incisor teeth |. Fibula articulating with the condyle of the calcaneum. 



4. No true alisphenoid canal: fibula ankylosed to tibia below; angle of mandible 



in the plane of the incisive alveolus. An iutertrochlear crest of humerus. 



Lagomorpha. 



These groups, as is well known, include families and genera which 

 display adaptations to various modes of life. Some are exclusively subter- 

 ranean, others are arboreal, and some live on the surface of the ground. Of 

 the latter, some are provided with formidable spines as a protection against 

 enemies, while others depend for their safety on their speed. Of the latter 

 character are the Leporidoi of the LagomorpM, and I have noted how that 

 they have superadded to the ordinary rodent structure certain points which 



