284 TEETH AND THE ALLIED ORGANS IN MAMMALIA. 
having 54. Some Cetacea (Homodonts) have the smallest 
known number ; the number in the Narwhal being apparently- 
reduced to two in the lower jaw, there being none in the 
upper. Among the Heterodonts, the Elephant has only two 
incisors in the upper jaw, and never more than 28 teeth in 
all ; the common rat has 20 teeth ; and in an Australian Rat 
there are only 12, there being one incisor and two molars on 
each side of the jaw. 
It is not possible to arrange the teeth of Homodonts in 
series and give them names. In a genus of Edentata 
(Dasvpus), a special tooth is implanted in the praemaxillary 
bone, and may be called an incisor; and the Two-toed Sloth 
has on each side a greatly developed tooth, which, however, it 
is difficult to identify with the canine tooth of Heterodonts. 
The names given to the groups into which the teeth of 
Heterodonts are divided must be regarded as arbitrary. The 
term incisors is applied to the front teeth, whatever may be 
their shape and function, which are, in the upper jaw, im- 
planted in the prasmaxilla. It must be remembered, however, 
that this connexion with the bone is only a secondary matter, 
the teeth originally belonging to the gums. Some teeth, 
indeed, are implanted in the suture between the prasmaxilla 
and maxilla. In nearly all placental Mammals, there are not 
more than three incisors above and three below on each side ; 
and the number is frequently even less. Excess over the 
typical number is very rare till we come to Marsupialia, where 
they may be four or five on each side. 
It has been supposed that, when the number of incisor 
teeth is reduced, the missing tooth is that on the outer side. 
The Pig has three incisors, the Babyroussa two, on each side ; 
and there is no doubt that it is the outer tooth which is want- 
ing in the latter. In the Camel and Llama, however, there 
is a single permanent incisor placed far back, which must 
correspond to the third. In Bears, there are ordinarily 'three 
incisors on each side ; but there is a genus in which the first 
in the upper jaw has disappeared. In the Sea-otter, also, the 
first incisor is wanting. The rule which has been referred to 
is, then, not absolute. 
No satisfactory definition of a canine tooth can be given, 
except that it is that which lies behind the intermaxillary 
suture, and that the lower tooth passes behind the upper 
one. 
The remaining teeth — molars — are divided into false and 
true, according to Owen. The false molars, or praemolars, 
replace the milk-molars ; the true molars have no pre- 
decessors. This definition must, however, be modified, as 
