G22 Dr. A. AniLiick-Cliristie-LiiiJe on the 



are to be found among tlicra, the presence of super- 

 numerary molars would not be unlikelV; but must^ on 

 the contrary, be expected ; as already mentioned, traces 

 of polyprotodonty have been found. 



3. In some living mammals the number of four molars is a 



rule. So it is iu the Marsupialia, at least according to 

 the general view. 



In an Insectivore, Centetes ecaudatus, the presence of 

 four molars is quite normal ^. 



I only mention this fact without entering into a dis- 

 cussion upon the homology of the teeth. 



4. In the literature I have found mentioned by Lechef one 



instance of an Insectivore, Erinaceus micropus, having a 

 fully developed tooth (Px) situated between P^ and M^ in 

 the upper jaw. Having stated that in Erinaceus euro- 

 pceus the free deep end of the tooth-band is swollen like 

 a bud on the lingual side of P"* (/. c. fig. 55), the author 

 concludes that the tooth Px iu Erinaceus microjjus has 

 been developed from such a lingual bud, and that, con- 

 sequently, this tooth is a fully developed premolar be- 

 longing to the postpermanent dentition, atavism being 

 out of the question. 



I cannot agree to this view. The presence of such lingual 

 buds does not necessarily prove that they will be developed 

 into functional teeth. So far as I know, the literature 

 does not mention any satisfactory ontogenetic evidence of 

 such a development of heterodont mammalian teeth. In 

 general these buds are early absorbed ; this is the case iu 

 the Soricidse and, according to Leche, also iu Erinaceus 

 europceus. 



As to the ontogeny of the tooth Px in question we know 

 nothing at all. Though the possibility of its belonging to 

 the postpermanent dentition cannot be denied, it is, however, 

 highly improbable ; at all events it is not proved. 



Another interjirctation is i)ossiblc, viz. that Px iu Erina- 

 ceus m'lcropus and Mx in Crocidura russula are homologous. 

 If thus the germ J\lx could develop into such a tooth as Px 

 and, vice versa, if Px has been developed from such a germ 

 as Mx, both these structures arc no doubt molars. Tiie 



* Thomas, 0., " On the Insectivorous Genus Eclduopsi, Martin, with 

 Kotes on the Deutitiou of the Allied Genera, " rrnc. Zoul. ISoc. Loudon, 

 1892 ; Dobson, G. E., ' A Monograph of the Insectivora, Systematic and 

 Anatomical,' part i. (London, l88J). 



t Leche, \V., I, c. p. 43, text-tig. 8. p. L")4. 



