AliSINOlTIIEElUM. 
21 
]->r()jection of the cingulum on its inner side. This tooth must have projected 
somewliat 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). 
Upper 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 lb. 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. 
Semi-dingrammatic figure of the upper milk-molars of Arsinoitherlum zitteli. 
a.c., anterior column ; a.e., antero-external cusp ; c., anterior part of cingulum ; c.', posterior part 
of cingulum ; p.c., posterior column ; p.r., postero-external cusp. About y 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. \ ., but is included in 
text-fig. 7. Its outer wall consists of parastyle, antero-external and postero-exteimal 
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. 
