518 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



of Cope and Osborn; and many a theory, more ingenious than suc- 

 cessful, as to the explanation of dentition would have been made 

 superfluous. * * * I shall try in a few words to give an idea of 

 what Winge's theory aims at.^ In the reptiles from which mammals 

 are descended the teeth were not used for chewing, only for catch- 

 ing and holding or rending the prey that was usually devoured 

 whole. True mastication of food seems to have originated within 

 the mammalian group and this involved a further development, 

 partly of the chewing muscles, partly of the teeth. Of special im- 

 portance is the appearance in mammals of an entirely new muscle, 

 the masseter, which through its attachment somewhat in front on 

 the lower jaw, lends increased strength to the movements of the jaw. 

 During the chewing and cutting process the lower jaw is moved 

 not only straight up and down, but also slightly to the sides, 

 and forward and backward, thus bringing each of its teeth alterna- 

 tively into contact with the two teeth of the upper jaw between 

 which it fits when the chewing apparatus is at rest. These contacts 

 stimulate the teeth to the development of secondary cusps, a process 

 which chiefly affects the more posterior teeth, that is, the ones 

 which receive the muscle power most strongly. Whereas in reptiles 

 the teeth are simply conical, they are in the oldest mammals three- 

 cuspidate (not counting some aberrant forms). The middle cusp is 

 the largest and the primordial one, the two others are the new addi- 

 tions due to contact stimulation. As an inheritance from the rep- 

 tiles, the lower jaw is slightly narrower than the upper jaw. Hence 

 the teeth of the lower jaw must slide along the inner side of the 

 teeth in the upper jaw. The act of chewing will thus have the 

 greater effect on the outer side of the lower teeth and on the inner 

 side of the upper teeth. Through the continuation of this influence 

 the three-cusped primitive mammalian teeth are further stimulated 

 to the formation of new cusps, as a rule two of them, which, accord- 

 ingly, appear on the outer side of the lower teeth and on the inner 

 side of the upper teeth. Whereas the lower teeth, set as they are 

 in the narrow movable mandible, do not as a rule develop any 

 further, still another pair of cusps may appear on the inner side of 

 the upper teeth, which are broadly supported in the solid maxilla. 

 * * * At the same time the three original, primitive mammalian 

 cusps may be reduced, being less used, and, at last, they may com- 

 pletely disappear. The various cusps now being numbered, though 

 in Winge's system and not in strict accordance with their phylo- 

 genetic succession (the primordial cusp bears the number 2), we 

 have a means through which we can indicate, with full certainty, 



1 As this paper in the Enjlish version ia intended for a somewhat different class of 

 readers than that to which the original Danish text was addressed, this paragraph has 

 been slightly enlarged, with Dr. Mortensen's consent, by Mr. Qerrit S. Miller, jr. — Ed. 



