12 Prof. Owen on the Class Mammalia. 
tooth, destined to succeed one into which the matrix has been 
converted ; such a tooth, therefore, when completed and worn 
down, is not replaced. The Sperm Whales, Dolphins, and Por- — 
h. In. the © 
Armadillos and Sloths, the want of generative power, as it may 
be called, in the matrix is compensated by the persistence of the — 
poises are limited to this simple provision of teet 
matrix, and by the uninterrupted growth of the teet 
n most other Mammalia, the matrix of the first-developed * 
tooth gives origin to the germ of a second tooth, which some- — 
times displaces the first, sometimes takes its place by the side of 
the tooth, from which it has originated. 
All those teeth which are displaced by their progeny are 
called ‘temporary,’ deciduous, or milk-teeth; the mode and di- 
rection in which they are displaced and succeeded, viz. from — 
bove downwards in the upper, from below upwards in the — 
lower, jaw, in both jaws vertically—are the same as in the Croc 
odile; but the process is never repeated more than once in any 
mammalian animal, A considerable proportion of the dental — 
series is thus changed; the second or ‘permanent’ teeth having — 
a size and form as suitable to the jaws of the adult, as the ‘tem- 
porary’ teeth were adapted to those of the young ani: 
The teeth between them and the canines are called ‘ remolars;’ 
they push out the milk-teeth that precede them, and are usually — | 
of smaller size and simpler form than the true molars. 
Thus the class Mammalia, in regard to the times of formation — 
and the succession of the teeth, may be divided into two groups, — 
monophyouonts,* or those that generate a single set of teeth ; and 
the diphyodonts,+ or those that generate tavo sets of teeth. 
this dental character is nut so associated with other organic char: i. 
acters as to indicate natural or equivalent subclasses. 
In the Mammalian orders with two sets of teeth, these organs _ 
acquire fixed individual characters, receive special denomin& 
tions, and can be determined from species to species, This indi- 
vidualization of the teeth is eminently significative of the hig 
grade of organization of the animals manifesting it. 
Originally, indeed, the names ‘ incisors,’ ‘canines,’ and ‘mo 
lars,’ were given to the teeth, in Man and certain Mammals, %& 
in Reptiles and Fishes, in reference merely to the shape and 
offices indicated by these names; but they are now used as arbl 
trary signs, ina more fixed and determinate sense. In § 
* udvo;, once; piv, I generate; ddodz, tooth. e 
t ols, twice; péw aud ddods. See “ Philosuphical Transactions,” 
nal. 
Those permanent teeth, which assume places not previously — 
occupied by deciduous ones, are always the most posterior 1 
their position, and generally the most complex in their form. — 
he term ‘molar’ or ‘true molar’ is restricted to these teeth. — 
