AND A DENTAL NOTATION. 
493 
The class Mammalia presents, in fact, two primary conditions of the dental system, 
according to which it might be divided into, — 
1st. Those that generate one set of teeth, or the ‘ Monophyodonts*,’ and 
2nd. Those that generate two sets, or ^DiphyodontS'|~.’ 
The ‘ Monophyodonts ’ include the orders Monotremata and Bruta (or the Edentata 
of Cuvier) and the Cetacea vera of Cuvier. 
The Diphyodonts include the Marsupialia, Rodentia, Insectivora, Cheiroptera, Ru- 
minantia, Pachydermata, Sirenia, Carnivora, Quadrumana and Bimana. 
I would not be misunderstood, however, as proposing this difference as a basis of 
classification : such dental characters are associated with too few corresponding 
differences of organization to lead to a natural binary division of the Mammalia. 
But, as regards the philosophy of the organs in question, considered in that class, the 
differences above enunciated form one of the highest generalizations, and the exigen- 
cies of clear and brief description require such general ideas to have their appropriate 
signs or names. 
The Diphyodont Mammals, then, are characterized by having a first set of teeth, 
commonly called the ^milk-teeth’ or ‘deciduous teeth,’ and a second set called the 
‘ permanent teeth,’ But the development of the latter, in relation to the milk-teeth, 
presents two modifications ; some of the permanent teeth are found in the same ver- 
tical line with the milk-teeth, push them out and take their place ; others are formed 
one after another behind the milk-teeth, in what may be called the same horizontal 
line, and come into place without pushing out any deciduous predecessors. Here, 
therefore, we have certain characters from development for particular teeth, and when 
to these characters are added others of equal constancy, derived from relative posi- 
tion, it will be readily understood how such characters, when clearly appreciable and 
firmly maintained through a series of comparisons, should enable the homologist to 
point out the very tooth in Man which becomes the great carnassial tooth in the jaw 
of the Tiger or the great complex grinder in that of the Wart-Hog. With respect to 
the accessory characters, one of the best is afforded by the relation of certain teeth to 
the constituent dentigerous bones of the complex jaws. Implantation in the premax- 
illary bone, or premaxillary part of the upper jaw, e. g., characterizes the tooth called 
‘ incisor,’ whatever be its shape or size ; and the true and constant character of such 
tooth being thus determined, the name ‘incisor’ becomes its arbitrary sign and loses 
all its primitive signification as descriptive of a particular shape or use. In like man- 
ner the tooth at the fore part of the maxilla, or the maxillary part of the upper jaw 
which coalesces with the premaxilla in Man, is called the ‘ canine.’ The molar series, 
according to the characters of development and succession above described, is divided 
into ‘milk-molars,’ ‘premolars’ and ‘true molars.’ The two latter kinds constitute 
the adult or permanent set of molars. Now these, in the diphyodont mammals, do 
not exceed ^^=28, i. e. seven on each side of both jaws. In the marsupial Diphy- 
* Mdvos once ; (pvw T generate ; ccoiis, tooth. 
•f A'ls twice ; cpvw and 6?ovs. 
