Vv EEE OER AMYVeUS Nig 
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 Hehidna, and the hard palate does not extend 
so far back as in that genus. The brain of this genus is 
smooth. 
The discovery of the real teeth of Ornithorhynchus only dates 
from the year 1888, when they were found by Professor Poulton ! 
in an embryo. Later Mr. Thomas found’ that the teeth persist 
1 
Fig. 55.—Duck-billed Platypus. Ornithorhynchus anatinus. x1. 
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 some- 
thing to do with the shedding of the latter. The hollows and 
1 Quart. J. Micr. Sci. xxix. 1888, p. 353. si 
2 Proc. Roy. Soc. xlvi. 1889, p. 127. See also Stewart, Quart. J. Mier. Sci. xxxiii. 
1892, p. 229. 
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