32 



E. M. MUSEUM MEMOIRS. 



In the lower jaw of Loxolophodon we find in the last molar, which has a structure 

 typical of the whole series, an outward-opening V, formed by two transverse 

 limbs, and behind this a prominent transverse crest. The upper teeth also 

 present an outward-opening V, formed of two limbs, as well as a posterior crest. 



Homologies of the Teeth. More in detail, the teeth present a likeness to the 

 Perissodactyle ungulate type in the uniting of the inner and outer tubercles by 

 the transverse Hmbs of the V. In the lower jaw these crests unite on the inner 

 side, forming a V opening outwards, here distinguished from the inward-opening 

 V of the Perissodactyle type. A more important distinction arises from the fact 

 demonstrated in the posterior molar of the lower jaw, that the posterior tubercle 

 on the inner side of the typical tooth has moved forwards and become placed on 

 the posterior slope of the larger anterior tubercle. This posterior tubercle grows 

 less marked as the teeth are followed forwards. It shows, however, that the 

 posterior limb of the V is homologous with the posterior transverse crest of the 

 Perissodactyle type of tooth, and that the posterior ridge of Loxolophodon is an 

 additional structure, probably derived from the basal cingulum and not from any 

 portion of the Bunodont type of tooth. In the Perissodactyle type, represented 

 by Lophiodon, the transverse ridges, not uniting on the inner side, form two for- 

 ward-opening crescents, which grind against the backward opening crescents of 

 the upper molars. In Loxolophodon the transverse ridges of the lower jaw are 

 directed obhquely across the tooth without assuming the crescentic pattern. In 

 the upper jaw, however, the anterior limb of the V forms a backward-opening 

 crescent, while the posterior limb is straight. The accessory tubercle on the 

 inner side of the apex of the V in the true upper molars is taken to represent the 

 fifth tubercle occasionally found in the Bunodont type. The posterior ridge of 

 the upper molars seems to be homologous with the same ridge in the lower jaw, 

 but does not attain so great a development. In each jaw it probably represents 

 the upgrowth of the posterior basal cingulum. 



Emphasis may be laid upon the presence of the second tubercle on the 

 inner side of the median crest of the lower jaw of Loxolophodon. It substantiates 

 the inference that the two limbs of the V are homologous with the two trans- 

 verse crests of the Perissodactyle type of tooth. If this is true of the lower it 

 is probably also true of the upper jaw, and the objection raised by Cope' that 

 the accessory tubercle prevents a comparison of these ridges with those of 

 Hyrachyus and Rhinocerus is not sustained. This is on the ground that the pos- 

 terior inner tubercle has probably disappeared, as in the premolars of the lower 

 jaw, and the accessory tubercle of the upper jaw represents the fifth tubercle 

 found in such a form as Acltanodon among the Bunodont types. 



•Extinct Vertebrata of New Mexico. Wheeler's Survey, Vol. IV., p. 192. 



