32 ARKIV rÖR ZOOLOGI. BAND 3. N:o 3. 



that the prisms originate from thin-walled canals or tubes 

 the shape of which are not at all or very little regular. This 

 origin of the prisms from tubes with thin walls explains alsa 

 quite easily the shape of the former. It is only the result of 

 tl^e mutual pressure of one tube on the other during the 

 growth, before the walls ha ve löst their original plasticity. 

 It is thus a parallel phenomenon to the oi^e seen in the 

 hexagonal cells of the honey-comb. 



Thanks to the different stages exhibited by the different 

 members of the milk-dentition and the permanent dentition 

 the structure of the teeth of Orycteropus is thus quite di- 

 stinctly explained. The crowns are rudimentary and within a 

 short time completeJy Avorn off. The functionary teeth of 

 Orycteropus are thus the homologues of the roots of former 

 teeth and of the roots of normal mammalian teeth. This 



helps also to explain the bicolumnar 

 shape of the molars of Orycteropus. 

 O. Thomas has shown that the hind- 

 most milk praemolar has two roots. 

 The same has evidently been the case 

 with the molars as well. When the 

 roots of the teeth during the phylo- 

 Fig. 12. Hindend of a tooth genetic development of these animals 

 of about the same shapo as orradually assumed the function which 



fig. 9 c. About 13 times ^, • i i . .i i> 



eniarged. otlierwise belongs to the crowns or 



normal teeth, and in correspondence 

 herewith became permanently growing. each root must as- 

 sume a columoar shape as is seen in the prsemolars. The 

 two roots of each molar growing si de by side were then 

 as a result of this development also firmly soldered together 

 so that the present shape of the molars was attained. Their 

 double origin is more or less pronounced outwardly, but 

 when the molars get very much dried up as is the case in 

 museums, the two columns often break completely apart thus 

 still more showing the primary constituents. 



In such a manner, as it appears, every peculiarity in 

 structure and shape of the tooth of Orycteropus can be ex- 

 plained and the seemingly insurmountable barriers derived 

 from the dentary system of the Tuhulidentata which appeared 

 to exclude them from all other Mammalia are broken down 

 to a reasonable height. It may therefore now be time to look 



