DENTAL SYSTEM 21 
and, what is far more general, permanent teeth may have no pre- 
decessors in the milk series. 
The complete series of permanent teeth of most mammals forms 
a complex machine, with its several parts adapted for different 
functions,—the most obvious structural modification for this purpose 
being an increased complexity of the individual components of the 
series from the anterior towards the posterior extremity of such 
series. Since, as has just been said, the complete series of the milk 
teeth often presents structurally and functionally a similar machine, 
but composed of fewer individual members, and the anterior of which 
are as simple, and the posterior as complex as those occupying 
corresponding positions in the permanent series,—and since the 
milk-teeth are only developed in relation to the anterior or lateral, 
never to the most posterior of the permanent series,—it follows 
that the hinder milk-teeth are usually more complex than the teeth 
of which they are the predecessors in the permanent series, and 
represent functionally, not their immediate successors, but those 
more posterior permanent teeth which have no direct predecessors. 
This character is clearly seen in those animals in which the various 
members of the molar series are well differentiated from each other 
in form, as the Carnivora, and also in Man. 
In animals which have two sets of teeth the number of those 
of the permanent series which are preceded by milk-teeth varies 
greatly, being sometimes, as in Marsupials and some Rodents, as 
few as one on each side of each jaw, and sometimes including the 
larger portion of the series. 
Although there are difficulties in some cases in arriving at a 
satisfactory solution of the question, it is, on the whole, safest to 
assume that when only one set of teeth is present, this corresponds 
to the permanent teeth of the Diphyodonts. When this one set 
is completely developed, and remains in use throughout the 
animal’s life, there can be no question on this subject. When, on 
the other hand, the teeth are rudimentary and transient, as in the 
Whalebone Whales, it is possible to consider them as representing 
the milk series; but there are weighty reasons in favour of the 
opposite conclusion.! 
Arrangement, Homologies, and Notation of Teeth.— The teeth of 
the two sides of the jaws are always alike in number and character, 
1 This and other questions concerning the homologies, notation, and suc- 
cession of the teeth of mammals are more fully developed in two memoirs by one 
of the present writers :—‘‘ Remarks on the Homologies and Notation of the Teeth 
of the Mammalia,” in the Journal of Anatomy and Physiology, vol. iti. p. 262, 
1869 ; and ‘“‘ Notes on the First or Milk Dentition of the Mammalia,” in the 
Trans. Odontological Society of Great Britain, 1871. See also an important 
memoir by Oldfield Thomas on the ‘‘Homologies and Succession of the teeth 
in the Dasyuride,” Phil. Trans. 1887, pp. 443-462. 
