Development of the Tetth of the SorlciJie. 613 



Sorcx — is enormous in size and l)ell-sliapcd (PI. XV III. 

 fij;. 2) . The free deep end of the tooth-band forms a bud-like 

 swelling on the lini^ual side. But in front of this tooth 

 there is a well-developed part of the tooth-band, showing 

 two swelling-* whieh. seen from the inner side, arc distinctly 

 ditl'crciitiatcd from the tooth-band (PI. XVUl. lig. 2). I 

 regard these swellings as germs of two sup[)rcssed anterior 

 incisors, I._, and I;,, and as probably homologous with the teeth 

 designated in the same way in Sorea^. I3 is about to be 

 cup-shaped. Transverse sections through these structures 

 arc here figured (tcvt-figs. 7 & 8). 



Fiff. 8. 



^' 





Xeomys fodiens. — Foetus (17 miu.). Lower jaw. Transverso section, 

 Fig. 7. I,. Fig. 8. I3. X 2-10. The rigjit side of the figure repre- 

 sents the lingual and the left the labial side of the jaw. 



From a comparison with Sorex araneus it q,ppears that ii) 

 Neonn/s a reduction has taken place with respect to the 

 anterior middle part of the tootii-band as well as to the 

 number of the incisors, no trace of Ii being distinguishable 

 in Neomys. 



A slight but definite thickening of the tooth-band repre- 

 sents the canine. As there is no such tooth in the upper 

 jaw at this stage, it is probable that the upper canine is 

 more early suppressed than the lower one. In Sorex the 

 case seems to be quite the reverse. 



On account of the above facts we Itave to horaologize the 

 following four antemolar germs as premolars. The full set 

 of premolars is thus at the foetal stage present in the lower 

 jaw in Neomys, as is the case in the upper jaw in Sorex. Pj 

 and P4 are well developed, the former being cup-shaped, the 

 latter bell-shaped. The free deep end of the tooth-band i^ 



