624 On the Development of the Teeth of the SoriciJse. 



Myosorex varius is thus the only living shrew wliicli lias 

 three premolars in the lower jaw. 



My investigation has put beyond a doubt that the Soricidae 

 have two dentitions, the permanent and milk dentitions. 

 Besides these dentitions traces of a third, i. e. a prelacteal 

 dentition, have been found in Surex araneus. 



Comparing the three genera examined — Sorex, Neomys, 

 Crocidura — with eaeh other, we find that as regards the 

 tooth-development they form a series, Sorex being the most 

 primitive and Ci^ocidura the most advanced form, Neomys 

 holding an intermediate position. 



In conclusion, it may be asserted that the results of this 

 investigation are also applicable to such other Insectivores 

 as are genetically connected with the Soricidse, and that they 

 are of importance not only with regard to the order Insecti- 

 vora, but also to the subclass Monodelphia, as 1 have shown 

 that the Soricidse are with respect to the teeth to be 

 regarded as intermediate forms between the polyprotodont 

 mammals — the Marsujnalia included — and the Monodelphia. 



EXPLANATION OF THE PLATES. 



Plate XVIII. 



Fiy.l. Sorex araneus. Foetus (11 mm.). Upper jaw. Tootli-band and 

 germs of all antemolars in the left half of the jaw (except the 

 canine), and germs of the tir,-;t three incisors in the right half of 

 the jaw and milk predecessors. lieconstruction of the ecto- 

 dermal parts. Ek, ectoderm ; Zl, the swollen free deep end of 

 the tooth-band on the lingual side of I'. A dotted line shows 

 the corresponding structure on the other side. The tooth-germ3 

 of the left half of the jaw are seen from the labial, tliose of the 

 right half from the lingual side of the jaw. x 70. 



Fiy. 2. JSeoniys fodiens. Foetus (17 mm.). Lower jaw. Tooth-band 

 and germs of all antemolars in the left half of the jaw, with 

 milk predecessors. Ileconstructiou of the ectodernuU parts. 

 ^^A;, ectoderm ; Z/ (upper), hibial ledge ; Zl (lower), the swollen 

 free deep end of the tooth-band on tlie lingual side of I4, (Pait 

 of Ij has been removed to show the germs of Ij and I3 and the 

 swollen free deep end of the tooth-band.) The tootli-gei-ms are 

 seen from the labial side of the jaw. x 70. 



Fiy. 3. Crocidura nis.tu/a. Fu^tus (12'; mm., stage I.). Upper jaw. 

 Tooth-band and germs of tlie premolars and the first molar in 

 the left half of the jaw and milk predecessors, lieconstruction 

 of the ectodermal ])arts. Ek, ectoderm ; M.r, tooth-germ 

 belonging to the milk-dentition. The tooth-germs are seeu 

 from the labial side of the jaw. x 70. 



