LÖNNBERG, STUDY OF THE VARIATION OF EUROPÉ AN BEAVERS. 13 



as the other Danube Beaver does not show anything of this 

 kind it must be considered to be the result of an individual 

 variation and this proves how little confidence can be put in 

 the structural pattern of the molars of the Beaver and how 

 httle valne it has for taxonomic purposes. 



For the full understanding of the different enamel patterns 

 exhibited by the different skulls of Beaver described above 

 it is very useful to study a very young skull with little worn 

 teeth. A suitable object for such a study is found in a small 

 not half grown skull of a female Beaver (killed in Jemtland 

 1829) belonging to the Natural History Museum in Stockholm. 

 The greatest total length of this skull is about 106 mm. Its 

 m^ have cut the gum but are hardly worn, the other molars 

 are of course worn but not much. Its p^ of both sides ^ and 

 m^ of the left side show already the typical pattern with an 

 inner and three outer enamelfolds. Fig. 5 shows how m^ 

 and 771- of the right side look (and m^ of 

 the left is similar to that of the right). 

 In m' of the right side there is anteriorly 



a träns verse enamel fold which is fully t^^-T^T^ r""^ , 

 aetached trom the other portion of the (to the left) and m^ (to 

 enamel system of the worn surface. This *^'® ""'l^'*^ .^^. "" ^^"^y 



r 1 ^ 1 . -, , , " young Swedish Beaver. 



transverse fold can be said to have been 

 formed by the fusion of the inner and the antero-external 

 fold and a similar structure has been recorded above from 

 different skulls of Beaver. The posterior portion shows the 

 middle and the posterior outer folds of a normal molar; the 

 middle fold is long and extends transversely över the surf- 

 ace so that it almost touches the enamel covering of the 

 inner surface of the molar. If this middle fold should fuse 

 with the inner enamel covering there would originate 

 another transverse and separate fold, just such a one as is 

 seen in the figure of ml The enam-el pattern of m^ consists 

 of two completely separate transverse enamel-folds and 

 behind them a V-shaped fold including the posterior outer 

 fold of a normal molar of an adult Beaver. As m~ is 

 less worn than m^ the enamel pattern of the former repre- 

 sents of course, an ontogenetically earlier stage than that of 



^ These p^ belong to the milkdentition and have two outer and an 

 inner roots. They should probably have been shed within a short time if 

 the animal had Ii ved. 



