392 Remarkable Lengthening in 



form of a chisel, or, when the upper and lower incisors are 

 brought together, that of a pair of scissors ; which is so 

 requisite for cutting the grass, as in rabbits, or for gnawing, 

 as in rats and squirrels, &c. Now, if, from a portion of one 

 of these teeth being broken off, or from a dislocation or 

 fracture of the jaw, or from any other cause, the teeth are 

 prevented meeting, so as to undergo the necessary friction, 

 the growth of the opposite tooth being at the same time con- 

 tinued, it must pass the other, so as to be quite beyond 

 its action; and might, supposing time allowed, be con- 

 tinued to any length. Hence I do not suppose that these 

 growths arise from any morbid condition of the teeth them- 

 selves. 



Each grinder, or molar tooth, in the hare (and, I should 

 suppose, in other completely herbivorous Rodentia), is com- 

 posed of lamina? of harder enamel and softer bony substance ; 

 and these teeth are of the same size throughout ; are convex 

 anteriorly, to give them a firmer hold in the jaw; and grow 

 from a pulp, as do the incisors : but I am not aware of the 

 same unnatural growth having been observed in the grinders, 

 perhaps from its being more difficult to prevent the necessary 

 attrition.* In the more omnivorous Rodentia, as the rats and 

 squirrels, the molares [grinders] are not continually growing, 

 but are knobby, covered with enamel, and fixed in the alveolar 

 processes by fangs. 



[The Dentition of the Elephant."] — While upon the subject 

 of continued developement of teeth, it may be interesting to 

 give a short account of the dentition in the elephant. 



The tusks of this animal (after the milk tusks have been 

 shed) continue to grow from a pulp, as do the incisors in 

 Rodentia. This is easily proved by the circumstance of iron 

 and leaden balls being often found embedded in the substance 

 of the tusks. The formation of the molares, or grinders, dif- 

 fers materially from this. The molar tooth, when it first cuts 

 the gum, is formed of perpendicular laminae or plates covered 

 with enamel, and joined by interstitial layers of bony sub- 

 stance; the whole tooth being covered by a bony crust, which 

 gives it smoothness : this latter substance, being much softer, 

 becomes worn away more quickly than the enamel, and gives 

 to the tooth the appearance of its being ribbed. The hard 

 laminae are formed first ; and, when covered by their proper 

 enamel, are joined together, beginning at the fore part, by the 

 bony substance. When the anterior layers of the tooth are 



* Mr. Lukis has (p. 22.) described, and there, in fig. 3., has shown, an 

 instance of some elongation in the grinders of the rabbit : I have described 

 a second instance in p. 24. — J. D. 



