Dr. C. A. Kofoid oa Platydorina. 555 



tion, and the word colony can be applied to it only by the 

 license of usage and as a matter of convenience. 



Reference has been made frequently in the preceding pages 

 to the prevalence of colonies whose symmetry has been dis- 

 turbed by loss of cells. In most instances only the empty 

 secondary sheath remains, giving no clue to the cause of the 

 loss of its contents. In collections made in Phelps Lake, 

 Havana, 111., in August 1896, however, colonies were often 

 found which were parasitized by one of the Sporochytriaceaj, 

 which upon examination proves to be Dangeardia mamillata, 

 described by Schroder (1898) as a parasite of Pandorina 

 morum. As these infested colonies frequently showed a loss 

 of one cell or more and exhibited all stages in the destruction 

 of the cell, it seems probable that the loss was due to the 

 parasite. Eudorina elegans and Pandorina morum occurred 

 in the same collection and were similarly infested. Two 

 additional genera, Platydorina and Eudorina, are thus to be 

 added to the list of hosts of Dangeardia. 



For the convenience of systematists a brief statement of 

 the generic and specific characters of the form herein described 

 is now given, followed by a key to the genera and species of 

 the Volvocinas for the assistance of students of this inter- 

 esting and not uncommon group of freshwater organisms. 

 Species not as yet reported to my knowledge from Illinois are 

 indicated by an asterisk when found elsewhere in this con- 

 tinent and by a dagger when not as yet reported from it. It 

 is not at all improbable that all the species here listed will 

 yet be found within this State. 



Platydorina, gen. nov. 



Colony flattened, the two faces compressed so that the cells 

 of the two sides intercalate ; flagella upon both faces on 

 alternate cells. Anterior and posterior poles of major axis 

 differentiated by the arrangement of the cells and by the 

 structure of the envelope. Long and short transverse axes 

 differentiated by the flattening of the colony. Cells similar, 

 biflagellate, each with stigma, chromatophore, and pyre:ioid. 

 Asexual reproduction by repeated divisions of all of the cells, 

 each forming a daughter colony. 



Platydorina caudata, sp. n. 



Colony flattened, horseshoe-shaped, twisted about one 

 eighth of a turn from right over to left; cells 16 or 32, 



