8 



ARKIV FÖR ZOOLOGI. BAXD 7. N:0 2. 



..-.cb 



ca vi ty, and it can sometimes be seen on tlie surface of rorucb 

 worn teeth as well. 



It is, liowever, difficult to express any definite opinion 

 about this fissure and its origin, whether it migbt be inter- 

 preted as a weakness in the centre of the tooth or more cor- 

 rectly of tbe radical portion of the same, since it consisted of 

 two roots, or not. If it was not such 

 an inheritance it appears that it would 

 have been most natural that the fissure 

 as a continuation of the pulp should have 

 extended in a plane which was parallel 

 to the greatest extension in breadth of 

 the tooth, or especiaJly to that of the 

 dentine. As the whole tooth as well as 

 the dentine is laterally compressed it thus 

 could have been expected that the fissure 

 would have been parallel to the jaw, and 

 not direeted at right angle, to the same 

 extending across the narrowest part of 

 the dentine, as it does. 



A microscopical examination of sec- 

 tions of teeth of the White whale in dif- 

 ferent stages of wearing reveals very 

 plainly to which changes the single teeth 

 are subjected during the life and growth 

 of the animal. A longitudinal section of 

 a young quite unworn tooth (conf. fig. 3) 

 shows that the ' small crown consists of 

 dentine with a very thin enamel covering 

 the thickness of which only amounts to 

 0,0 3 9 mm., and that the dentine occupies 

 the central portion of the root of the 

 tooth as well. The exteriör layer of the 

 root counted from the abruptly thickened 

 collar at the base of the crown and all the way to the basal 

 end consists of cement. Broadly speaking the tooth thus is 

 formed by two hollow cylinders, an inner of dentine which 

 is closed at its apical end which is conical and thinly capped 

 with enamel, and an outer of cement which is open at both 

 ends. The dentine is not fully continuous on the section but 

 shows different layers of different density, as it appears. 



Fig. 3. Longitudinal 

 section through a tooth 

 of a qviite young White 

 whale showing the dif- 

 ferent layers of dentine 

 and cement, e enamel. 

 d dentine, c cement- 



