ho 
tute less than a seventh of the total representatives of the species. 
Their distribution throughout the year (Table I.) accords with the 
results obtained by Lauterborn (’98), who found that the average 
length of the posterior spine from January to May and from October 
to December was from 78 to 48 y, while from June to September it 
was from 28.5 to 21. In Table I. it will be seen that the longer- 
spined forms which I have referred to A. cochlearis var. macracantha 
and var. typica occur in the plankton from January to May 31, and 
then disappear, returning again, in small numbers, October 25. The 
short-spined variety referred by me to A. cochlearis var. stipitata 
and the spineless var. tecta are, on the other hand, continued during 
the summer. The natural result would be that the average length 
of the spines in the species as a whole would fall during the summer 
months. It is apparent that this tendency on the part of A. 
cochlearis to become shorter and smaller during the summer months 
does not bear out the contention of Wesenberg-Lund (’98) that 
winter individuals are smaller and summer ones larger among 
perennial rotifers. He reports var. tecta as ‘‘die Hauptform des 
Winters’ in several Danish lakes, and the variety with a long 
horn as a summer form, found in July—August. 
Of these varieties, macracantha, typica, and stipitata intergrade 
in our waters with numerous connecting links, while var. tecta 1s not 
connected with the other forms by many individuals with inter- 
mediate characters. Lauterborn (’98) also notes the greater inde- 
pendence of this variety in the waters of the Rhine. 
In Table I. the seasonal distribution of these three vanienes the 
long-spined (typica and macracantha), the short-spined (sé¢pztata), 
and the spineless (tecta) are given separately. It will be noted that 
the long-spined form has the distribution above mentioned, that 
var. tecta runs throughout the whole year, and that var. stpitata is 
absent in midwinter and isa common summer form. The relative 
numbers of the varieties fluctuate in different years. For example, 
var. tecta was relatively but one fourth as abundant in 1897 as in 
1898. As shown in Table I., whenever coincidently present in the 
plankton all the varieties respond to the causes which produce the 
rhythm of occurrence, the rise, culmination, and decline of the pulses 
being much alike in all of the varieties. 
About three eighths of the females noted in 1898 were ovigerous, 
carrying as a rule but a single egg. Instances of two eggs were 
