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, 
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, 
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 
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 
