308 HOUSE, GARDEN, AND FIELD 



ness of their constituents, keep always sharp ; teeth with 

 rounded studs for crowns ; pavement- teeth ; and folded 

 teeth, with ridges and hollows of unequal hardness, so 

 that they never wear smooth. The angler-fish has hinged 

 teeth, which bend inwards easily, but cannot be forced 

 outwards, and detain the struggling prey as in a trap. 

 The pike has the whole mouth and gullet crowded with 

 teeth. The male narwhal has only one functional tooth, 

 but this is several feet long half the length of the 

 body. 



Not less various are the situations in which teeth are 

 developed. The edges of the jaws are the places usually 

 chosen, but the roof of the mouth is often armed with 

 teeth also. Some fishes, which swallow their prey whole, 

 have backward-pointing teeth, projecting from all parts 

 of the mouth. Even the gill-arches, bony and jointed 

 hoops, primarily intended to spread out the gill-filaments, 

 are made to bear teeth, and the last gill-arch in many 

 fishes loses its respiratory character altogether, becoming 

 transformed to a single or double tooth-bearing plate 

 which underlies the gullet. The upper parts of other gill- 

 arches may also expand into broad tooth-bearing plates 

 steadied by attachment to the skull. In some fishes the 

 lower plate plays upon the upper one, and forms together 

 with it a pharyngeal mill, able to grind up the food. Where 

 a highly peculiar instinct calls for the development of teeth 

 in a quite unexpected place, the adaptation of some struc- 

 ture originally intended for a different purpose may be of 

 startling singularity. Wherever there is epiderm, or its 

 equivalent, enamel can be developed ; wherever there is 

 derm (inner skin) or its equivalent, dentine can be de- 

 veloped, and these two things are the ordinary com- 

 ponents of teeth. But the tooth must also have a sup- 

 porting base, and it is here that the greatest ingenuity 

 is displayed. 



A certain African snake, the Dasypeltis, or " egg-eater " 

 of the Cape Colony, lives upon eggs. This food is no 



