] development oj ike Teeth of the SoiicklfC. Q'lW 



pivscncc of these tcetli would couscqucutly be an instance oL" 

 atavism. 



AVlietlier the tooth-rudiment ^Ix in Croctdnra russula is a 

 mohir or premolar must, however, be left undecided. At all 

 events, the presence of this tooth-yerm leads us back to the 

 Mesozoic inaiiimals, which have a {^^''cater number of molars 

 (and premolars) than now living licterodont mammals (with 

 the exception of M;/riiU'Cobii(s) . 



In this connection it may be pointed out that if it could 

 be definitely shown that Mx (which tootli-rudiment evidently 

 belongs to the same dentition as the milk-premolars, and has 

 })robably been absorbed at the same time as these teeth) is a 

 suppres-sed molar, then the view that the molars belong to 

 the milk-dentition is fully confirmed. 



As has been shown above, tlie genus Sorex in its adult 

 stage has the full set of premolars in the upper jaw. In the 

 lower jaw a complete set of these teeth is never present, all 

 known Sorieidee, Mith the exception of one species, having 

 only two lower premolars. But, as ap|iears from the above, 

 the Soricidse surely once had four premolars also in the 

 lower jaw. This seems to be confirmed by palseontological 

 evidence. In Protosorex crassus Scott * has shown the 

 presence of four minute teeth between the molars and the 

 large procumbent incisors. The first and the fourth are 

 larger than the other two. Unfortunately he gives no figure 

 of them and expresses no opinion as to their homology. 



Comparing Scott's description with the result of my in- 

 vestigation, I daresay that the four minute teeth in question 

 in Protosorex crassus in all probabiliftj rejjresent the four 

 premolars. 



In Myosorex variiis Dobson f has pointed out the ])resence 

 of a rudimentary minute tooth situated between the second 

 and the third lower teeth, which he regards as a probable 

 canine, thus existing in a single species. 



As a result of my investigation I venture to maintain that 

 this tooth is not a canine, but that it is the second premolar, 

 which remains, though in a rudimentary condition, in the 

 adult Myosorex varius, but in Sorex, Neomys, and Crociduru 

 is found only in foetal specimens. 



* Scott, W. B., " A new Insectivore from the White Hiver Beds," 

 Pi-oc. Acad. Nat. Sci. Philadelphia, vol. xlvi. (1894). 



t Dobsou, G. E., /. c. 1890, part iii. fasc. 1 ; " Mandibular Deutition of 

 the Shrews,'' Jouru. Auat. Physiol, vul. xx. (I880-86). 



