492 
PROFESSOR OWEN ON THE DENTITION OF PHACOCHCERUS, 
in both, to the fourth premolar, and to the first, second and third true molars of the 
typical dentition of the placental Diphyodonts. 
These explanations will serve to render the symbols of the remaining phases readily 
understood by those who may not have studied the principles of dental notation 
which I communicated to the ‘British Association’ in 1848, and have more fully 
exemplified in the article ‘Teeth’ of the ‘Cyclopaedia of Anatomy and Physiology;’ 
their utility will be obvious when they are found to express, in a few lines, facts in 
Comparative Anatomy which would require almost as many pages if recounted by 
ordinary description. 
This system of anatomical notation is the practical fruit of the discovery or deter- 
mination of a type or common pattern of dentition to which the teeth of a certain 
proportion of the Animal Kingdom could be referred, and of a concomitant attain- 
ment of the power to trace a particular tooth under every modification and disguise 
of size and shape, throughout the different species of those animals. Every tooth, 
thus capable of being individualized and determined, merits and, for the purposes 
of description, requires to have a proper name, and can be signified by a symbol, 
which is still more convenient for those purposes. 
The dental system manifests this regular and determinable character in a large 
proportion of the mammalian Class; but not in any animal of inferior organization. 
Like the definition of a species, the definition of a tooth or other part of an organism 
becomes possible only when its characters are constant and definite, and is easy in 
proportion as those qualities are exalted. The definition of a mineral species is more 
difficult than that of a vegetable species, and the definition of a species of low cellular 
plant is more difficult than that of the highly organized dicotyledon. In proportion 
as wholes rise in the scale of nature or of life, their recognition and definition be- 
comes easier, and the like obtains also of their parts. As animals ascend in the scale 
of complexity their organs and parts become more definite ; and homologies are 
more extensively, easily and satisfactorily determinable. 
We cannot point out in one species of Echinus the answerable spine of any given 
spine in another species, nor can we determine the homology of a tufted foot from 
one species of the many-jointed Annelides to another ; but in insects each particular 
leg may be determined through all its modifications of form and function throughout 
the class. So, likewise, with the teeth : the same individual tooth cannot be traced 
from fish to fish, or from reptile to reptile ; the teeth in the cold-blooded classes differ 
too much in their number in different individuals, and too little in their development 
and succession, to yield the requisite characters to the homologist who keeps his 
faculty of comparison under due control. In those Mammalia, likewise, as e, g. 
the Cachalots, Dolphins and Armadillos, in which the teeth are very variable in 
number and often very numerous, but without any definite order of shedding and 
replacement, no particular tooth can be identified and traced from one species to 
another. 
