LÖNNBERG, THE DENTITION OF DELPHINAPTERUS LEUCAS. 11 



as each new layer of dentine (and the with the same corres- 

 ponding external layer of cement) projects basally a little 

 beyond its predecessor. 



The diameter of the cylinder of dentine measures about 

 5 mm. the increase of these parts is thus not very great when 

 compared with the quite young tooth. 



The cement which constitutes the exteriör part of the 

 tooth is also stratified, although in a less conspicuous manner 

 than the dentine. It has become considerably thicker than 

 in the quite young tooth. 



A section through a much worn tooth of an old animal 

 (basicranial length about 60 cm.) is quite in accordance with 

 the description above, in spite of the much greater size of 

 this tooth, which had a diameter of fully 14 mm., or nearly 

 twice as much as the tooth reproduced in fig. 4. The den- 

 tine is arranged in similar layers between which in some cases 

 a stratum of odontoblasts is seen, but not alvays. The ce- 

 ment is likewise stratified. There seems, however, to be a 

 certain difference with regard to the microscopical texture of 

 the cement of this big tooth compared with that of the 

 3^ouno"er teeth as it is highly interwoven with fibrils, which 

 on a longitudinal section appear to run chiefly in a radial 

 direction, although they are not, by far, regularly arranged 

 but sometimes turned more in one and sometimes more in 

 another direction. The presence of these fibrils indicates that 

 these cementlayers in reality consist of ossified connective 

 tissue. It is evident that by means of these fibrils the firmity 

 of the tissue is highly increased, and this is more needed in 

 the large teeth of the old than in the small teeth of the young 

 animals, because the cement plays a much more important 

 part for the individual tooth in the former than in the latter. 

 This statement is easily verified by measuring the thickness 

 of the different layers of tissue and if this is done it will be 

 found that the transversal diameter of the central cylinder of 

 dentine has exactly the same length, viz. 5 mm. in this large 

 tooth as in the younger tooth described above, in spite of the 

 great difference between the two in total size. It is thus only the 

 growth of the exteriorly surrounding layers of cement which 

 has effected the increase in size of the larger tooth. Consi- 

 dering how the interiör cylinder of dentine is fitted in into the 

 exteriör tube of cement it is also easily understood that the 



