TEETH OF PLATYPUS 



113 



teen pairs of ribs and only two luml>ar verteljiae. The skull is 

 expanded in front, and the bill is supported by two, at first 

 diverging, and then converging, premaxillae. Between them is 

 the fajmous " dumb-bell shaped l)one," wliich is l)elieved to be the 

 representative of the reptilian prevomer. The pterygoids are 

 smaller than in EcJiulna, and the hard palate does not extend 

 so far back as in th;it genus. The brain of this genus is 

 smooth. 



The discovery of the real teeth of OrmthvrJh't/Hchus only dates 

 from the year 1888, when they were found by Professor Poulton ^ 

 in an embryo. Later ]\Ir. Thomas found '" that the teeth persist 



Fig. 55. — Duck-billed PlatypiT^. Or»ifhorhi/nrhusanatinns. x ^. 



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 



^ Quart. J. Micr. Sci. xxix. 1888, ]i. 353. 



- Proc. Roy. Soc. xlvi. 1889, p. 127. See also Stewart, Quart. J. Micr. Sci. xxxiii. 

 1892, p. 229. 



VOL. X ■ I 



