1896.] NATURAL SCIENCES OF PHILADELPHIA. 369 



upright on a horizontal plane, the chord of the arc described by 

 the exposed incisors forms an angle to the horizon of 45 to 50 de- 

 grees, and the radius of this arc is about one-fifth ( l^o) the basilar 

 length of the skull. Roots of lower incisors extending to base of 

 pra. 2. Incisive foramina terminating opposite pm. 1 ; widest at 

 or near base. Palatal bridge shorter than greatest width of in- 

 cisive foramina. Palatine foramina opposite middle of pm- 3 . 



Meaaurements.'^^ — Average of four adults: hind foot 147 millime- 

 ters ; ear, from crown, 98. Skull : total length 102 ; greatest 

 breadth 48 ; greatest diagonal length of nasal 41 ; greatest length 

 of mandible 75 ; greatest breadth of mandible, 49 ; alveolar length 

 of upper molar series 19. 



General remarks. — The peculiar incisor dentition of this species, 

 so far as I have been able to compare it with other members of the 

 genus Lepus, is quite unique, not only in the obliteration of the sul- 

 cus of the upper anterior pair but in the extension of the roots of 

 both upper and lower incisors, the former being planted far behind 

 the inferior anterior maxillary border and the latter reaching the 

 bases of the second premolars. 



Externally grwnlandicus will probably not be found to differ 

 materially, even in its summer dress, from typical northern arctieus. 

 Pabricius, whose experience was mainly confined to southern Green- 

 land, twice asserts that its summer coat does not change in color 

 from that of winter. Whether green landicus will prove to be en- 

 tirely distinct from the hares inhabiting Iceland and the extreme 



ous tooth than anything else." 



While a formation analogous to this structure is seen in some adult speci- 

 mens of all the species of Polar Hares I have examined, in no case does it as- 

 sume the prominent and functional character which it invariably attains in 

 adult gnenlandi.cus. In the others it manifests itself as a liomogeneous de- 

 posit along the bottom of the sulcus ; in the Greenland animal it is a laminate 

 bistriate structure, having its inception near the base of the tooth in a honey- 

 combed hastate flap which lies within, but does not touch the sides of the sul- 

 cus and which, as it extends toward the crown of the tooth, increases in den- 

 sity and calibre and is closely cemented within the groove. Un the exposed 

 surface of the incisor it often overtops the contour of the face of the tooth 

 and widens up on the tooth face in the form of a protuberant lamina, with 

 from one to three irregular longitudinal striae upon its enamel-like surface. 



In nearly mature foetal specimens of gnenlandicus there is not the slightest 

 indication of this incisor groove layer. In young gra:nlandicus, one month 

 old, the cementum has begun to form closely along the bottom of the groove 

 and reaches along the median third of its length to the alveolar edge of the 

 premaxillary. At this period its consistency is that of indurated cartilage. 

 In specimens apparently but lately arrived at maturity, the sulcus is partly 

 filled to the tip, and in very old skulls the groove is obliterated, as described 

 above. 



^^For measurements of type gr«nlandicus, see tables, pp. 374, 375. 



