136 
OK THE TEETH OF MAMMALIA. 
responding teeth of the other jaw. All the teeth of the Carnivora , and the 
incisors of Ruminantia , have the crowns alone covered with enamel, and not 
running into their substance. Graminivorous animals have a third ingredient, 
viz. crusta petrosa , which I adverted to before, entering into the composition of 
their molar teeth. This substance is harder than bone, and softer than enamel, 
and as the teeth of these animals suffer a great deal of attrition, the three sub¬ 
stances coming into action at the same time, and being of different degrees of 
hardness, wear unevenly, and thereby keep a rough surface constantly on the 
tooth; whereas if one substance alone were engaged, it would soon become 
smooth, and the tooth would then be nearly useless. The three substances are 
best seen in a vertical section of an Elephant’s tooth. It will then appear that 
the crusta petrosa is the most external, and entirely surrounds the others ; it is 
of a brownish colour. Next comes the enamel, and most internally lies the bone 
or ivory. Owing to the pulps of the Elephant’s tooth being separated at the 
top, each process of bone is surrounded first by enamel, but as that is limited in 
thickness, the remainder of the interval between the pulps is filled up with 
crusta petrosa , which is in larger quantity than either of the other substances. 
It thus fills up all the hollows which would otherwise be present in the tooth, 
and which would be very inconvenient, as affording lodgment for particles, of 
food, &c. The provision for making up for the wear of the Elephant’s grinders 
is very curious ; only one grinder and part of another can be seen at any one 
time protruded through the gum of the animal. The anterior one is gradually 
worn away by constant mastication ; its fangs and alveolus are then absorbed, the 
posterior tooth coming forward to supply its place. This process is repeated 
seven or eight times during the life of the animal, each new one being larger than 
th® one preceding it. This accounts for the teeth of Elephants always appearing 
more worn at the anterior than at the posterior part. Respecting the molar 
teeth of some of the Rodentia I may refer to the first volume of The Naturalist , 
page 48. 
In this hasty description of teeth in general, I have purposely avoided going 
into the details relative to the numbers of each kind of teeth to be found in the 
various animals I have alluded to, as that subject is fully explained in all works 
on the classification of animals. 
