ture, and method of attachment. They are almost entirely used for prehension, as nearly all fishes, like reptiles and birds, do not masticate but gulp down their food as quickly as possible. The exceptions, however, are interesting. Some rays and the cestracion of Australia have the jaws filled with teeth, flattened and joined together like blocks in a pavement, or like mosaic. These are used to crush sea-weed and mollusks. In a former geological age, this was the prevailing form of teeth in the whole order of sharks. The common carp has its teeth on the bones of the pharynx, and hence masticates its food in its throat. For this purpose the food is sent back after being swallowed. Some fishes are toothless, but most fishes have hundreds of teeth, frequently covering all parts of the mouth. The teeth of fishes and of reptiles are shed and replaced indefinitely.
Fig. 7.—Skulls and Teeth of Muskrat (Fiber zibethicus). Size reduced. 1, cranium with upper incisors overgrown, due to loss of lower incisors; 2 a, skull and jaw with normal teeth; 2 b, lower incisor removed, to show its great length.
Reptilian teeth present no great variety. Toads, tortoises, turtles, and some lizards are entirely destitute of teeth. Frogs have teeth in the upper jaw. Those of serpents assist in swallowing the huge prey. The poison-fangs of venomous species present a peculiar and complex modification. They are fastened to movable bones which are worked
