136 ON THE TEETH OF MAMMALIA. 



responding teeth of the other jaw. All the teeth of the Carnivora, and the- 

 incisors of Euminantia, 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 df 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 inteival 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 work* 

 on the classification of animals. 



