m. 2.] 



COPE ON MIOCENE RODENTIA. 



363 



Many of the above genera stand in evident genetic connection with 

 existing forms. The Miocene Castors doubtless inckide the ancestor of 

 the modern beaver. The Iscliyromys is a primitive type of the Sciuridce, 

 and GymnoptycJius connects it directly with the existing forms by the 

 character of its molar teeth. Eumys is the primitive form of Hesperomys, 

 as Paciculus is of Sigmodon. JEntoptyehus and Pleurolicus are the near 

 ancestors of the Geomyidcc of the Pliocene and present periods. Palcco- 

 lagus, PanolaXj and Lepus form a direct genetic line. The ancient genera 

 all differ from their modern representatives in the same way ; that is, 

 in the greater constriction of the skull just posterior to the orbits and 

 accompanying absence of postorbital processes. This relation may be 

 displayed in tabular form as follows : 



Skull "Wider behind orbits. 



Skull narrower behind orbits. 



Postorbital processes. 



No postorbital processes. 



Postorbital processes. 



Ko postorbital processes. 



Sciurus. 

 Lepus . . 



Castor fiber. . 

 Hesperomys . 



Castor peninsulatus. 



Ischyromys 



Eumys 



Palffiblaffus 



ISTone of this species of this fauna are of larger size than their mod- 

 ern representatives. In the cases of the beaver, squirrels, and rabbits, 

 the ancient species are the smaller. 



SCIUHOMOEPHA. 



SCIUEUS Linn. 



lu this genus the molars are f or f , the first superior small when 

 present. The grinding surfaces of the crowns when unworn present in 

 the superior series a single internal cusp, which is low and antero-pos- 

 terior. From this there extend to the external border of the crown two 

 low transverse ridges, whose exterior terminations are somewhat en- 

 larged. In the lower jaw the transverse ridges are not visible, and there 

 is a low tubercle at each angle of the crown, between which there may 

 be others on the border of the crown. Attrition gives the grinding sur- 

 face of the latter a basin-like character. The foramen infraorhitale is a 

 short, narrow fissure, situated in the inferior j^art of the maxillary bone 

 in front of its tooth bearing i)ortion, but descending nearly to the level 

 of the alveolar border. 



The well-known characters of this genus are found in the mandibles 

 of species which I obtained from the White River Miocene beds of Colo- 

 rado and Oregon. The teeth display the subquadrate form of this 

 genus, without any tendency to the transverse enlargement seen in 

 Arctomys, Cynomys^ and Spermophilu/i. Two of the species, S. vortmani 

 and S. rcUctus, are as large as our gray and red squirrels, respectively, 

 and the third, S. ballovianuSj is about the size of the Tamias quadrivittatus. 



