teen pairs of ribs and only two lumbar vertebrae. The skull is expanded in front, and the bill is supported by two, at first diverging, and then converging, premaxillae. Between them is the famous "dumb-bell shaped bone," which is believed to be the representative of the reptilian prevomer. The pterygoids are smaller than in Echidna, and the hard palate does not extend so far back as in that genus. The brain of this genus is smooth.
Fig. 55.—Duck-billed Platypus. Ornithorhynchus anatinus. × 1⁄6.
The discovery of the real teeth of Ornithorhynchus only dates from the year 1888, when they were found by Professor Poulton[1] in an embryo. Later Mr. Thomas found[2] that the teeth persist for a considerable portion of the animal's life, and are only shed, like milk teeth, "after being worn down by friction with food and sand." We have already (p. 98) called attention to the general similarity of these teeth to those of certain of the earliest Mammalia and of mammal-like reptiles. The teeth are all molars, and they are either eight or ten in number. They are replaced by the horny plates of the adult animal; but the mode of replacement is curious. The plates are developed from the epithelium of the mouth, but round and under the true teeth; the epithelium of the mouth grows gradually under the calcified teeth, a method of growth which has possibly something to do with the shedding of the latter. The hollows
