14 ARKIV FÖR ZOOLOGI. BAND 5. N:0 6. 



the latter. A still earlier stage is exhibited by the almost 

 unworn m^. Anteriorly it has a transverse ridge correspon- 

 ding to the anterior transverse enamelfold of m^, behind this 

 in the middle an obliquely transverse ridge ^ with the same 

 situation and direction as the middle transverse enamelfold 

 of m^ to which it corresponds, and finally in the posterior 

 outer portion of the crown two tubercles connected by a lower 

 ridge which together when worn would form such a V-shaped 

 enamelfold including the posterior outer fold as in m^. 



If these facts are put together it is easy to understand 

 and explain all the different enamel patterns exhibited by 

 the Beavers in different ages and different stages of wearing 

 even the anomalous ones which all are due to the folding of 

 the enamel organ. This folding takes place partly from the 

 crown and partly in continuation and correspondence with 

 this, from the sides. The former may for shortness sake be 

 termed the vertical folding, the latter the lateral folding. 

 When only the products of the vertical folding are worn the 

 enamel pattern of the molar is such as exhibited by m^ on 

 fig. 5, m^ of the same figure shows a but little låter stage. 

 This explains why independent transverse folds are seen 

 chiefly in youngish animals, but if the vertical folding has 

 been deeper than usual such a pattern may persist even in 

 låter stages and cause such anomalies as has been recorded 

 above. The anterior vertical fold of the young molar is con- 

 tinued towards the base of the molar by a lateral folding on 

 either side, viz. the one producing the antero-external fold 

 and the other producing the inner fold of the normal adult 

 molar. — By this is explained why the transverse detached 

 enamel fold may be said to be produced by the fusion of the 

 inner and antero-external fold. — The second vertical folding 

 is continued towards the base only by a lateral folding on 

 the outer side of the tooth, viz. the one producing the middle 

 outer fold of the adult molar, but this one cuts deeper into 

 the tooth than the other lateral folds. The vertical fold 

 between the two posterior tubercles of the crown of the young 

 molar is continued towards the base by the lateral folding 

 which produces the posterior outer fold of the adult molar. 

 The inner fold and the middle outer fold are continued further 



^ This and the anterior ridge have in an unworn stage a row of small 

 tubercles along the free edge. 



