234 LEPORID^— LEPUS 



not circular, but broadest transversely, and filled with dentine. 

 The first upper premolar has on its anterior face at least three 

 enamel, cement-filled foldings, of which the central one is deep, 

 the two outer shallower. The second, third, fourth, and fifth 

 teeth have each, on the inner side, a single deep re-entrant enamel 

 fold, extending about three-quarters of the distance across the 

 tooth ; its sides are nearly in contact, and more or less crenated. 

 The enamel lines the sides of the transverse folds, and com- 

 pletely invests the internal faces of the teeth ; externally it has 

 atrophied, so that the outer faces of the teeth are formed in 

 great measure of bare dentine. For an account of the evolution 

 of this structure, see above, under Ochotonidcs, pp. 156-158. 

 The last upper molar is a small, greatly reduced tooth, elliptical 

 in section and quite simple in structure. 



The lower cheek-teeth are framed on the same plan as the 

 upper, but in these the enamel is infolded to a much greater 

 extent, so that they are divided into two portions, an anterior 

 and a posterior, by a single transverse fold from the outer side. 

 In accordance also with the well-known principle, that the inner 

 side of an upper corresponds with the outer side of a lower grinder, 

 and vice versa, the enamel is developed continuously on the outer, 

 and atrophies on the inner sides. The anterior portion of the 

 first grinder {pm 3) has a small re-entrant angle on its anterior 

 face, and a broader one on its outer side. The second, third, 

 and fourth have their posterior portions with lateral diameters 

 about four-fifths of those of the anterior portions. The last is 

 small, and resembles a double cylinder with a larger elliptical 

 anterior and a smaller separate rotund posterior portion. 



The mandibular tooth-rows lie so much closer to each other 

 than those of the maxillaries that only the teeth of one side are 

 capable of opposition at the same time, and the jaws have con- 

 sequently a lateral motion in mastication. 



In the skeleton the cervical vertebrae are uniformly lengthened, 

 the transverse processes do not project laterally from their 

 centra to a marked extent, and the anterior and posterior 

 spines are comparatively elongated. 



The lumbar vertebrae have wide and long transverse pro- 

 cesses with expanded extremities, which arise abruptly from 

 the anterior half of their centrum. Those of the fifth are the 



