No. 2.] THE HARD PARTS QF THE MAMMALIA. 247 
it would get would probably stimulate the posterior part of the 
crown (heel) of the inferior molar until it should rise to meet 
the internal part of the superior. The more rapid wear of the 
paraconid and of the hypocone would bring about the general 
contact of the two series of molars in animals of mature age. 
The increased development of the heel of the inferior molar 
under these circumstances may have brought about the atrophy 
of the paraconid by appropriation of material of nutrition. But 
this explanation is hypothetical only. At present there is none 
other to offer. 
10. THE ORIGIN OF THE LOPHODONT MOLARs. 
Quadritubercular molars have their tubercles either simple or 
modified into crests; to the two conditions have been applied 
the terms dunodont and lophodont. The component tubercles, 
or crests, of each tooth, may be either opposite to or alternate 
with each other; these conditions are termed anztiodont and 
amebodont, respectively. Thus all quadritubercular molars 
may be classified under four heads, as represented in this dia- 
gram :— 
Antiodont. Ameebodont. 
Bunodont . O33 | Sea pike ne aS as 
Lophodont Aa Dat aS, ee 
Two principal kinds of relation between the teeth of opposite 
jaws also exist, the zsognathus and anisognathus. In the former 
the teeth of opposite jaws are of equal width, and the jaws them- 
selves oppose each other; in the latter the teeth of the lower 
jaw are narrower than those of the upper, and the branches of 
the lower jaw embrace a narrower transverse space than the 
upper jaws and palate. 
The problem presented by the Mammalian quadritubercular 
dentition is that of the derivation of the various lophodont or 
crested types from the simple and primitive bunodont types. 
It is necessary to remember, in the first place, that teeth of 
the quadritubercular type no longer alternate with each other 
in the vertical direction ; that is, those of the inferior series no 
longer occupy the spaces between those of the superior when 
the jaws are closed. In the vertical sense they oppose each 
other. This has been accomplished by the development of the 
fourth tubercle, forming an inner posterior angle of the superior 
