GIO Dr. A. Arnback-Cliristie-Linde on the 



Summary. 



1. During the foetal life the tooth-band forms a continuous 



curve in both jaws, the connection of the halves in tlie 

 middle remaining until a late stage of development, even 

 until calcification has begun in the teeth of both denti- 

 tions; in the lower jaw traces of the tooth-band are 

 found even in the young Sorex. 



2. The most striking feature in Sorex araneus is the presence 



of five incisor-germs in the upper jaw and four in the 

 lower jaw, a greater number than has hitherto been 

 found in any heterodout monodelph. Even if it should 

 n(.t be admitted that the structure F, which is but little 

 differentiated, is a tooth-germ, I think there can be no 

 doubt about the nature of the structure designated by 

 me as 1^ (with the predecessor Id^). The presence of 

 three vestigial incisor-germs in the lower jaw may be 

 considered as proved (vide PI. XIX. figs. 1, 2), though 1 

 could not pursue the development of these germs on 

 foetal material. 



3. There is in the upper jaw a distinct rudiment of a canine, 



proving that the ancestors once had such teeth ; also in 

 the lower jaw traces of canines arc observable. 



4. The above facts enable us to homologize the minute ante- 



molar teeth and to ascertain that the number of pre- 

 molars is complete. It can now hardly be doubted that 

 Sorex has four premolars in the upper jaw, and once 

 had the same number in the lower jaw. 



5. A milk-dentition is present, consisting of the following 



teeth : — 



Id- Id^ Id = Cd rd^ Pd- Pd^ F d* 

 Idi PdiPda Pd.' 



Some of these tooth-germs calcify, viz. : — 



Id^ Pd^ Pd^ Pd' 

 Pdi Pd4* 



6. The milk-dentition is highly reduced and functiouless, 



in all probability disappearing before birth with the 

 exception of the lower \\\^, which remains in the young 

 Sorex. 



7 . Besides the milk and permanent series of teeth there is 



a trace of one more dentition, i. e. the prclactcal den- 

 tition, a minute growth of the tootli-band having been 

 found on the labial side of Id'". 



