536 UNGULATES. 



smaller third lobe in the GirafFe. The lower molars of the genus 

 Auchenia are peculiarly distinguished by the vertical ridge at the fore 

 part of the anterior lobe (PL 133, fig. 2, m, 2 & 3), which does not 

 exist in the Camels of the Old World. 



In all Ruminants the outer contour of the entire molar series is 

 slightly zigzag, the anterior and outer angle of one tooth projecting 

 beyond the posterior and outer angle of the next in advance. 



The premolars are smaller and more simple than the molars 

 with which they form a continuous series in the true Ruminants ; in 

 the upper jaw^ they are not divided into two lobes by an internal 

 cleft, but resemble a single lobe of the true molars, of greater breadth 

 than thickness, with a single central crescentic island, and usually 

 with an internal basal ridge. The central crescents have a more 

 complex contour in Megaceros than in Bos, and the first premolar, 

 which is always the smallest, is relatively larger in the Deer than in 

 the hollow-horned Ruminants. In the small Musk-deer the crescentic 

 enamel island is reduced to a small internal notch or fold, and the 

 outer border of the crown is trenchant and pointed. In the lower 

 jaw the premolars decrease in size from the third to the first, which 

 has usually a compressed conical crown with a sinuous inner 

 surface : the second and third premolars have two deeper notches 

 on the inner side, and a small second hinder lobe sHghtly marked off 

 by a vertical depression on the outer side of the crown. All the three 

 lower premolars have compressed, sub-trenchant and pointed crowns 

 in the small Musk-deer {Tragulus) : the true Musk (Moschus) 

 more resembles the ordinary Deer in its premolars. The aberrant 

 Camelida deviate most from the Ruminant type in their premolar 

 teeth. In Auchenia the first is soon shed; the second (PI. 133, 

 fig. 2, p 2) is unusually small and simple, and the third fp 3) 

 does not surpass in breadth a single lobe of the true molars, in either 

 jaw. In the Dromedary and Camel the second and third premolars 

 bear the same proportions to the molars as in the Llama and Vicugna, 



remains longer as in the last molar of the Giraffe, the great Irish Deer and the Sivatherium, 

 which plainly shows that the additional lobe in the last lower molar is a reduced analogue not 

 of half, but of a whole lobe, and that it is essentially composed of two half-cylinders, presenting 

 as long as its central enamel fold remains, two crescentic ridges, but of smaller size than those 

 in the two normal lobes which it succeeds. The essential distinction of the third lobe of the 

 last lower molar is that it has not a distinct root. 



