536 Lloyd Williams . — Studies in the Dictyotaceae . 
periodicity is clearly dependent upon the difference between spring and 
neap tides, it was not anticipated that the slight differences between one 
spring tide and another would be reflected in variations in the time of 
appearance of the different stages of the crops. But, unlikely as it may 
seem, it is here that we must look for the primary causes of the differences, 
while meteorological phenomena of various kinds may occasionally either 
restrain or intensify them. When looked at from this standpoint the 
results are so simple and consistent that it may be of some interest to 
state that all the data for the North Wales coasts had been collected 
and tabulated in chronological order before the right clue to their mean- 
ing was discovered. This makes the record more valuable than if the 
explanation had been in one’s mind while studying the facts. 
The tidal peculiarities here referred to are either differences of rise 
between successive spring tides, or differences in the intervals of time 
separating them from each other. 
1. There is generally a fairly regular alternation of higher and lower 
spring tides. Sometimes the higher ones occur at the New Moon, and 
the lower at the Full: at other times the reverse is the case. It is but 
rarely that the two spring tides occurring in a month are the same height. 
During the fruiting period of Dictyota , this inequality always obtains, 
and there is the further consideration that the equinoctial tides, which 
occur about this time, are higher than ordinary spring tides. Thus, to 
take two examples from the year 1899 : — 
The spring tide on August 9 was 18 ft. 4 in. 
„ „ „ ,» 25 „ 21 ft. 8 in. 
Again ., „ October 12 „ 18 ft. 11 in. 
While „ „ „ 27 „ 21 ft. 7 in. 
In the following discussion the spring tides will be designated simply 
‘ lower ’ or ‘ higher ’ springs, as the case may be, whether they are ordinary 
or equinoctial. 
2. The interval between two highest spring tides, corresponding to and 
very nearly coinciding with the interval between New and Full Moon 
(or between Full and New Moon), and technically called a ‘ semilunation,’ 
varies in length. In the tables it is sometimes as low as twenty-five tides, 
while at other times it is as high as thirty-one tides. 
Tables I and II give the most important of the data relating to 
these two groups of tides. The records of the other tides are fragmentary, 
but as far as they go they are in complete harmony with the results here 
quoted. 
The following are some of the most obvious deductions from the 
tabulated results. 
1. In the case of the higher spring tides of either August or September 
