26 
THE PERIODICAL CICADA. 
. AValsli-Riley enumeration of 1878 gave the records for sixteen broods, 
wliicli were designated by Roman numerals from I to XA'I, the enumer- 
ation being based on the se([uen('e of the difl'erent broods after 1868. 
In 1809, in his First Missouri lieport, Professor Riley, liavingin the 
meantime secured the manuscript paper of Doctor Smith, added the 
six broods from this paper not represented in the Walsh-Riley enu- 
meration, increasing the number of the broods to XXII, and renum- 
])ered them again in accordance with their sequence, beginning with 
1S69. These broods vary enormously in their extent, some of them 
being represented by scattered colonies, which perhaps have no real 
relationship in point of origin, and others covering nearly uniformly 
vast stretches of territory extending over several States together. 
Several are rather unimportant, or lack confirmation, and one of 
them. Brood III, was founded on an erroneous record and has been 
dropped. 
In the enumeration of the broods by Walsh-Riley, and later l)y 
Riley, the two races are mixed together and a sequence of numbers 
given which, after the first thirteen years, lost all significance as a 
record of the order of the broods in time of appearance, and from 
the first obscured the true kinship of the broods in each race. If, on 
the other hand, each race be considered separately and its broods be 
arranged in a series in accordance A\^th their sequence in time, an 
important natural relationship in point of origin and distribution is 
plainly indicated. 
Taking first tlie broods of the 17-year race, as Rile}' numbered them, 
it vdW be seen from the subjoined table tliat if the enumeration begin 
with Brood XI, the 17-year broods follow each other in regular succes- 
sion for eleven consecutive years, then after a break of one year fol- 
low Broods Y and VIII, and after another break of one year Brood 
IX. Another break of one year precedes the next recurrence of Brood 
XI, with which the series starts. 
Chronological order of the Riley broods of the Cicada from 1893 to 1910. 
Year. 
17-year 
race. 
13-year 
race. 
Year. 
17-year 
race. 
13-year 
race. 
1893 
XI 
XII 
XIII 
XIV 
XV 
XVII 
XIX 
XX 
XXI 
XVI 
XVIII 
II 
IV 
VI 
VII 
1902 
XXII 
I 
' "v "■ 
VIII 
* IX ■ " ■ 
189-1 
190? 
190-1 
1905 
190(1 
1907 
1908 
:::::::::: 
1895. 
.... 
189() 
1897 
XVI 
1898 
XVIII 
1899. . . . 
II 
1900. 
1909 
IV 
1901 
X 
1910 
XI 
VI 
Taking up the 1 3-year broods in the same way, it will be seen that 
if the enumeration start with Brood XYI, a 13-year brood follows in 
regular succession for six years. With the exception of the ver^' 
doubtful Jk'ood X, which is separated from the last 13-year brood by 
