308 



BULLETIN* MUSEUM OF COMPARATIVE ZOOLOGY. 



ure 14; but it is evidently out of the question that four cusps should 

 gain a systematic position of this kind, as in the center of Figure 15, 



Fig. 14. 



without some common control. Whenever such an arrangement is 

 found, special examination should be made of it. Again, while a two- 

 swing cusp is of common occurrence, a three-swing cusp, Figure 16, 



Fig. 16, 



must be rare, for it involves the intersection of three unrelated lines in 

 a systematic manner; and a four-swing cusp, Figure 17, is practically 

 an impossible occurrence. True, a three-swing cusp must be produced 



Fig. 18. 



at a certain stage of the change shown in Figure 18, where a sweeping 

 meander is undercutting its scarp and thus pushing one two-swing cusp, 

 A, towards another two-swing cusp, B ; for at a certain stage in the 



