91 
animal in their composition, and when they decline the organisms 
upon which the disappearing animals were feeding have an oppor- 
tunity to multiply with less decimation in their ranks. 
This species is one of the most abundant of the synthetic organ- 
isms in the summer plankton, and next to Euglena is the foremost 
among the synthetic elements of the food cycle of the plankton. 
The presence of many light-colored or even colorless forms (forma 
hyalina K1.) justifies the suspicion that members of this genus, like 
those of its near relative Euglena, adopt holozoic nutrition in the 
presence of abundant organic matter suitable for food. 
This species, as well as the others above listed, is exceedingly 
variable in the proportions of the lorica, in its color, and in the 
development of the neck. It is very desirable that its life history 
and the full limits of its variation be determined before many more 
new species are proposed in the genus. 
In addition to the forms above listed, the following have been 
noted as present in small numbers in the summer plankton, viz.: T. 
armata Ehrbg., T. caudata Ehrbg., T. torta Stokes, T. urceelata 
Stokes, and 7. volvocina var. rugulosa K1. 
Uroglena americana Calkins.—This species was found in small 
numbers in July and September, 1897, and in January, 1899. 
Uroglena radiata Calkins.—This species was found in January, 
1896; in April and May, 1897; and in March and April, 1898. There 
was a vernal pulse of 15,279 on April 29, 1896. 
Uroglena volvox Ehrbg.—This species was found sparingly in the 
spring plankton in 1896. Uroglena is one of the few organisms 
which the usual method of plankton collection and preservation 
fails to keep in fair condition for subsequent indentification. 
The gelatinous matrix is easily crushed, and debris adheres to 1t so 
as to obscure it beyond recognition. Judging from the frequency 
of Uroglena in the living plankton it is very probable that the genus 
is much more abundantly represented in the Illinois River than the 
data at hand indicate. The genus seems to prefer the cooler waters 
of autumn and spring to those of midsummer. 
Volvox aureus Ehrbg.—This species was found from March to 
August, but in small numbers and irregularly. 
Volvox globator L.—This was somewhat more abundant than the 
previous species, and was found more frequently, especially during 
