12 ARKIV FÖR ZOOLOGI. BAND 7. N:0 2. 



former hardly can expand beyond the boundaries once fixed. 

 This arrangement is, however, also of mechanical importance 

 for keeping the tooth in a conical sliape. It is clear that 

 wlien the tooth is subjected to wearing the somevvhat softer 

 cement will wear away more quickly than the härd er inner 

 cyhnder of dentine. In consequence of this the dentine will 

 protrude beyond the cement at the worn apical end of the 

 tooth, which accordingly assumes and retains the shape of a 

 blunt cone. The thickness of the tip or point of the tooth 

 stånds in direct correspondence with the thickness of the 

 inner cyhnder of dentine. A narrow inner cylinder of dentine 

 surrounded by a thick outer coat of cement wiJl thus when 

 worn result in a conical and comparatively more sharp-pointed 

 tooth than if the dentine had been thicker and more massive. 



From the description above it may be quite clear how 

 the teeth of the White whale are built, how they continue 

 to grow and by this substitute the worn off parts. The small 

 original crown is soon worn away, the easier as the enamel 

 covering is so very thin, and it is thus during the greater 

 part of the life of the animal the root which fmictions as 

 tooth ^ and keeps on growing in the manner described and ex- 

 plained above. 



If a question about the homology of the teeth of the 

 White whale is raised, it seems clear that none of the nor- 

 mally 9 teeth on either side as well of the upper as lower 

 jaw can be regarded as corresponding to incisors or canines 

 as none of them is inserted in the premaxillary nor in close 

 vicinity to the suture. The foremost of them must thus be 

 premolars, and the others also premolars and possibly molars. 



Through the investigations especially by Abel it has 

 been made evident that the increase in number of the 

 teeth of the Squalodons has taken place a t the posterior end 

 of the premolar series but in front of the molars. The hind- 

 most teeth in the jaws of these primitive whales, the true 

 molars — inherited from their ancestors — are pushed back- 

 wards by the new members of the dentition, that is the new 

 posterior premolars, and have a tendency of becoming reduced. 



^ This forms, oniitting all the great differences in structure etc, an 

 analogy to the condition prevailing in Orycteropus in which the present 

 anthor has proved elsewhere (»On a new Orycteropus from Xorthern Congo 

 and some remarks on the dentition of the Tubulidentata. Ark. f. Zoologi. 

 Bd. 3. n:o 3. Stockholm 1900) that the roots alone function as teeth since 

 the small crowns soon have been worn off. 



