458 CHORDATA. 
in the Monotremata, temporary teats only in the Mefatheria 
and permanent ones in the Eutheria. 
The evolution of mammary glands probably commenced before 
that of the viviparous habit. We can see how the ventral surface of 
the parent lent itself first for the incubation of the eggs and later for the 
tending of the young. The desire of the young for fluid would naturally 
be satisfied by the local glands, and if we may suppose that the duties 
of incubation and nurture were shared by both sexes, we can to some 
extent understand how the males of many mammals still have mamme 
and functionless mammary glands. After the viviparous habit was 
developed the male, divested of his share in the incubation, would 
gradually give up the mammary function as well. 
The mamme in modern mammals vary much in posi- 
tion, though all situated upon the ventral surface. They 
may be pectoral, axillary, abdominal or inguinal according 
to their position on the breast, under the armpit, along the 
abdomen and in the groin respectively. 
TrEETH.—The teeth are well developed in most mammals, 
though some types, such as whales, ant-eaters, Monotremata 
and others, appear to have lost them. The characters of 
mammalian teeth may be summed up in the expressions— 
thecodont, heterodont, diphyodont , to which we may add a 
limitation to a single row on the premaxillze and maxillz 
above and to the dentary below. In a ¢hecodon? dentition 
the teeth are held in definite sockets in the bone; a Aetero- 
dont dentition is one in which the teeth differ markedly 
amongst themselves in size and shape; and, lastly, in a 
diphyodont dentition there are two sets of teeth succeeding 
one another in the life of the individual. [We may recall 
that the teeth of most reptiles are fused to the bone 
(acrodont or pleurodont), they are usually of the same size 
(homodont), there are several series of teeth (polyphyodont), 
and the teeth often occur upon the palatines, pterygoids or 
vomers in addition to the premaxille and maxilla. The 
crocodile, as in other anatomical features, approaches the 
mammal in having one row of thecodont teeth which are 
slightly heterodont. ] 
DEVELOPMENT OF A TooTH.—A typical mammalian tooth arises 
from an examel-organ consisting at first of a protrusion of the mucous 
layer of the epidermis downwards into the dermis. This becomes 
pushed in on the under side and the dermis thus protrudes into it 
as a small ‘‘dentine-germ.” The mucous epithelium, bordering the 
