lyon] the hares and their allies 349 



and wide, represented by the American species and Ochotoiia 

 ladacensis. 



In the Ochotona roylii group all the available skulls have an oval 

 foramen between one and two millimeters in length in the anterior 

 and superior part of each frontal bone. This foramen is not seen 

 in any of the other skulls, although some of the American specimens, 

 notably 36298, 0. coUaris, Fort Yukon, Alaska, show light spots in 

 the anterior part of the frontal bone, evidently the equivalent of 

 the two above mentioned foramina. 



In Ochotona roylii the single large opening in the side of the 

 maxilla is less rotund than in the other species at hand, is more 

 elongated, and immediately beneath it there is a small amount of 

 fenestration, 



TEETH 



Leporid.e 

 (Plate XCI, 2-9) 



The dental formula of most of the Leporidse is I \, C ^, Pm |, 

 M f . In the genus Pcntalagus the molars are reduced to |. As 

 there are no differences in structure between some of the premolar 

 and the molar teeth, in what follows in regard to the teeth, the 

 premolars and molars taken together will be called the molariform 

 teeth. 



Incisors. — The first upper incisors are large and heavy, being 

 typical rodent incisors in general form. The second upper incisors 

 are minute teeth placed directly behind the first incisors. The 

 anterior surface of each front incisor is marked by a longitudinal 

 groove, extending more or less deeply into the tooth, and which 

 may or may not be filled with cement. 



The lower incisors are perfectly plain rodent incisors. 



Check Teeth. — All the upper molariform teeth are built on the plan 

 of a cylinder, filled up with cement and dentine. The cylinder, 

 however, is not circular in section, and the enamel undergoes certain 

 infoldings, except in the last tooth of the series. The greatest 

 diameter of each tooth is from side to side. 



The first upper molariform tooth has enamel variously infolded 

 on its anterior face ; most commonly there is a deep median reentrant 

 angle, and two smaller reentrant angles, one on each side of the 

 median one. On the other three faces of the tooth the enamel is 

 not infolded. 



The second, third, fourth, and fifth upper molariform teeth have 

 each a single deep reentrant angle on the inner side, the sides of 



