AESINOITHEEIUM. 



21 



projection of the cmgulum on its inner side. This tooth must have projected 

 somewhat below the others, and was separated from its fellow of the opposite side 

 by a considerable interval (about 2-5 cm. in the figured skull). 



Upjper Milk-dentition. — The upper milk-dentition is imperfectly known. The most 

 nearly complete specimen of what may be regarded as the upper milk-molars is 

 figured on PI. V- fig. 5 (also text-fig. 7). Mm. 4 is precisely similar in structure to 

 the permanent molars, though much smaller. Its anterior column is immediately 

 beneath the hinder border of the antorbital foramen. It is only slightly worn, 

 so that the anterior crescent (c.) of the cingulum is not yet reached. Mm. 3 is also 

 much like a permanent molar, but the compound character of the anterior column is 

 much more clearly shown than in the molars : it consists of an outer portion, probably 

 equivalent to the parastyle and the antero-external main cusp, and an inner portion, 

 the antero-internal main cusp. The form of the posterior enamel-covered surface of 



Text-fig. 7. 



inner face, 

 c -^ c 



ae. 



mmz 



r"^ mmi- Tnm.3 



Semi-diagrammatic figure of the upper milk-molars of Arsinoitherium zilteli. 



a.c, anterior column ; a.e., antero-external cusp ; c, anterior part of cingulum ; c.', posterior part 

 of cingulum ; p.c, posterior column ; p.e., postero-external cusp. About ^ nat. size. 



the column apparently shows a stage in the infolding of the ectoloph to which the 

 origin of the peculiar bilophodonty is ascribed above. In mm. 2 the ectoloph is 

 complete, and is only slightly concave, but it can be seen how a slightly deeper 

 infolding would give rise to a tooth like mm. 3. The inner part of the tooth consists 

 of two transverse crests, and in front of these a ridge of the cingulum, which with 

 the anterior crest encloses an enamel-lined pit as in the other teeth. The antero- 

 internal tubercle is large and tends to unite in wear with the postero-internal, so as 

 to form an inner wall similar to that found in the permanent molars. 



The first milk-molar is not shown in the figure on PI. V., but is included in 

 text-fig. 7. Its outer wall consists of parastyle, antero-external and postero-external 

 elements. On the inner face there is a trace of an antero-internal element only, but 

 the cingulum (c.) is very strongly developed, particularly posteriorly, where together 

 with the ectoloph it encloses a deep enamel-lined fosette. 



The milk-incisors are simple conical teeth. 



