AESINOITHEEIUM. 19 



presents somewhat the pattern of a reversed molar of some such type as that of 

 Lophiodon or Tapirus (PL V. fig. 7). At this stage of wear the inner face of the 

 tooth is covered with cement only, the enamel, as already mentioned, ceasing just 

 below the cingulum on that side, while externally it continues to a much lower level. 

 In the latest stages all that remains of the enamel is found on the posterior and 

 external surfaces of the main columns, round the pit marking the last remnants of the 

 main transverse valley and round that lying between the anterior crest of the cingulum 

 and the anterior main column (see PL V. fig. 8). It is by examining the molar in its 

 latest stages of wear, which give a section of the basal part of the tooth, that some 

 explanation of its peculiar character may be arrived at. In this condition (PL V. 

 fig. 8) the crown is seen to consist of an outer wall uniting the transverse crests, 

 which are likewise joined at their inner ends, while in the middle they are separated 

 by the remains of the main transverse valley : there is also a remnant of the fossa 

 lying betwieen the anterior main column and the anterior crest of the cingulum. 

 From the appearance of the outer wall it seems probable that the brachyodont tooth 

 from which these hypsodont molars have been derived had an ectoloph composed of 

 well-developed parastyle and mesostyle, together with the antero-external and postero- 

 internal main cusps, the inner portion of the tooth being constituted by the antero- 

 internal and postero-internal cusps, which tended to unite with the antero-external 

 and postero-external cusps respectively to form a pair of transverse ridges. The 

 ectoloph was deeply infolded in front of the mesostyle, and, as the height of the 

 tooth increased, tliis fold deepened rapidly till the tooth became nearly divided 

 into anterior and posterior columns. If this interpretation be correct, the anterior 

 column includes parastyle, antero-internal and antero-external cusps, while the 

 posterior consists of the mesostyle, postero-internal and postero-external cusps. 



The form of tooth from which these molars were derived was probably much like 

 that of the Hyracoids, in which the styles are strongly developed and the ectoloph is 

 deeply folded. Moreover, as will be shown, there is a strong tendency in the early 

 forms of that group for the teeth to form a continuous series. These peculiarities 

 may be in themselves of little importance, but they suggest that possibly 

 Arsinoitherium and the Hjracoidea may have originated from a common stock at 

 some remote period. At any rate, both seem to have arisen in the same region. 



The roots of the molars (PL V. fig. 8 a) consist of a pair of transverse plates 

 [p.r. and a.r.) extending the whole width of the anterior and posterior borders of the 

 tooth, and a much smaller and shorter root {m.r.), imperfectly divided into two and 

 situated on the inner border of the tooth between the inner ends of the main roots. 

 These latter seem to remain open for some time after the crown has come into wear, 

 so that the tooth is on the way to become rootless and permanently growing.' The 

 anterior main root appears to support the parastyle, the antero-external main cusp, 

 and the antero-internal crest of the cingulum. The posterior root supports the 



