DEVELOPMENT OF THE TEETH IN MAMMALIA 33 



Reptiles have usually teeth of a simple conical form, 

 while the Mammalia have multicuspidate teeth, and different 

 views have been held as to the manner in which the cusps 

 of the higher forms have been evolved. 



According to one view, first formulated by Ameghino (1) Con- 

 and strongly supported by Kiikenthal (13) and Rose (9), the 

 multiple cusped teeth of Mammalia have been produced by 

 the union or concrescence of single conical teeth united to 

 form a single tooth, made up of these separate elements, 

 each cusp of a compound tooth corresponding to the single 

 conical reptilian tooth. 



This union of the conical teeth might take place either in 

 the antero-posterior (mesio -distal) direction by the union 

 of teeth of the same series, thus shortening the jaw, or in 

 the transverse or bucco-lingual direction, as suggested by 

 Kiikenthal, by the union of the teeth of different series. This 

 has been called the concrescence theory. 



The basal ridge or cingulum is a ridge which surrounds the Cingulu 

 tooth at the neck, and is considered by many to play a very 

 important part in the production of cusps. Speaking of this 

 structure, Osborn (8) says : ' By its disappearance in some 

 regions and by its elevation into prominences in others, the 

 form of a tooth may be prof oundly modified, and it thus comes 

 to be regarded as a sort of mother of cusps.' As shown by 

 C. S. Tomes (27), the cingulum is well developed in most of 

 the Insectivora, where the crowns ( often bristle with sharp 

 points ', which are produced by the elevation of the cingu- 

 lum, and these points often exceed in length the principal 

 cusps of the tooth. Marett Tims considers that the develop- 

 ment of cusps from the cingulum explains to a great extent 

 the evolution of complex tooth forms. It would certainly 

 appear that accessory cusps arise from the cingulum. 



The Tritubercular Theory 



Professor Cope (4) in 1879 discovered a large series of fossil . 

 mammals in deposits of the Tertiary Age at Puerco Canon 

 in New Mexico. These small animals showed a general 

 similarity of form in the molar teeth, even in those of 

 apparently different habits. These teeth are made up 

 of * three main tubercles on the crowns of both upper and 



MUMMERY 



