Art. XV. — RevievF of the Rodentia of the ITIiocene 

 Period of ]¥orth America. 



By E. D. Cope, N. A. S. 



RODENTIA. 



Members of tliis order were very abundant during the White Eiver 

 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, Hes- 

 peromys, and Lepus. All of them occur in the Truckee beds, while the 

 first-named only has been found in the White Eiver formation. All of 

 the species belong to the three great divisions of the order which now 

 inhabit North America, while the fourth, the SystricomorpJia, which is 

 very sparingly represented on the continent, has not yet been detected 

 in the formations ia question. It appears in a single species of i^orcu- 

 pine in the Loup Fork beds. 



The four primary divisions of the order Rodentia are thus defined, 

 principally after Brandt and Alston. 



I. lucisor teeth, f . Fibula not articulating "with tlie superior condyle of the calca- 



neum. No intertrochlear ciest 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 zygomatic process." 

 An interj)terygoid fissure Hystricojiorpha. 



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 ; 

 coronoid process high, falcate. Fibula distinct from tibia. No interptery- 

 goid fissure Sciuromorpha. 



3. Maudible with the angular portion springing from the inferior edge of the 



sheath of the inferior incisor (except Bathyerginw). Fibula coossified with 

 the tibia. Malar short, usually supported on a maxillary process. No inter- 

 pterygoid fissure (except in Bathyergince) Myomorpha. 



II. Incisor teeth f . Fibula articulating with the condyle of the calcaneum. An in- 



tertrochlear crest of humerus. 



4. No true alisphenoid canal ; fibula ankylosed to tibia below ; angle of man- 



dible in the plane of the incisive alveolus Lagomorpiia. 



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

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



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