SUCCESSION OP TEETH. WHALEBONE. 173 



only present in the latter, but are largely developed, serving as 

 weapons of attack and defence ; as in the Boar (rig. 100). 



184. In the Mammalia in general, as in Man, the teeth are 

 not much developed at the time of birth, that they may not 

 interfere with the act of sucking; and they do not make 

 their appearance above the gum, until the time approaches 

 when the young animal has to prepare its own food, instead 

 of simply receiving that which has been prepared by its 

 parent. The teeth which are first formed are destined to be 

 shed after a certain period, and to be replaced by others. 

 They are called milk-teeth ; and in Man they are twenty in 

 number, namely, four incisors in the front of each jaw, and 

 two canines and four molars on each side. These begin to fall 

 out at about the age of seven years ; previously to which, 

 however, the first of the permanent molars appears above the 

 gum, behind those of the first set. The incisors and canines 

 of the first set are replaced by incisors and canines respec- 

 tively ; but the molars of the first set are replaced by teeth 

 like small molars, having only two fangs ; these are called 

 false molars, or, more properly, bicuspid teeth (fig. 93). The 

 second of the true molars does not make its appearance until 

 all the milk-teeth have been shed ; since it is only then that 

 the jaw becomes long enough to hold any additional teeth. 

 The third does not usually come up until the growth of the 

 jaw is completed ; and as this time corresponds with that at 

 which the mind as well as the body is matured, they are 

 commonly known as wise or wisdom teeth. There are then 

 thirty-two teeth in all, or sixteen in each jaw ; namely, four 

 incisors, two canines, four bicuspid, and six true molars. In 

 extreme old age, these teeth fall out like those of the first 

 set ; but they are not replaced by others, and their sockets 

 are gradually obliterated. 



185. There are a few Mammalia which do not possess teeth. 

 This is the case with the common Whale, in which they are 

 replaced by an entirely different structure. From the upper 

 jaw (fig. 102) there hang down into the mouth a number of 

 plates of a fibrous substance (fig. 101), to which we give the 

 name of whalebone, though it is really analogous to the gum 

 of other animals. The fibres of these plates are separate at 

 their free extremities, and are matted (as it were) together, so 

 as to form a kind of sieve. Through this sieve the Whale 



