422 THE VERTEBRATE SKELETON. 



the better are its milk teeth developed, and the more do they 

 form a reproduction on a small scale of the permanent set. 

 This fact is well seen in the Primates, Carnivora and Ungu- 

 lata. The method of notation by which the dentition of any 

 mammal can be briefly expressed as a formula has been already 

 described. The regular mammalian arrangement of teeth for 

 each side is expressed by the formula 



i - c - pm - m x 2 ; total, 44. 

 o 1 4 o 



MONOTEEMATA. Iii Echidna teeth are quite absent. In 

 the young Ornithorhynchus 1 functional molar teeth of a multi- 

 tubercular type resembling those of some Mesozoic mammalia 

 are present, but in the adult they disappear, their office being 

 discharged by horny plates. 



MARSUPiALiA 2 have a heterodont dentition, which has 

 generally been regarded as almost monophyodont, the only 

 tooth which has an obvious deciduous predecessor being the 

 last premolar. The researches of Rose 3 and Kiikenthal 4 tend 

 to show that the teeth of Marsupials are developed in the 

 same way as in other mammals, and are diphyodont. In 

 the case of the premolars, teeth which are homologous with 

 the permanent teeth of other mammals begin to develop as 

 lateral outgrowths from the milk teeth, but afterwards be- 

 come absorbed, so that the teeth which actually persist belong 

 to the milk series. The last premolar, however, does as 

 a rule develop and replace its milk predecessor; sometimes, 

 however, as in Didelphys, it takes its place among the milk 

 molars without replacing one of them. 



1 See E. B. Poulton, P. R. S., Feb. 1888, and Quart. J. Micr. Sci., 

 Vol. xxix. 1889; also Oldfield Thomas, P. E. S., XLVI. (1889). 



2 W. H. Flower, Phil. Trans., vol. 156, pp. 631 641, 1867 ; also 

 Oldfield Thomas, Phil. Trans., pp. 443462, 1887. 



3 C. Rose, Anat. Anz. vn., p. 639. 



4 W. Kiikenthal, Anat. Anz. vi., p. 364. 



