On Platydorina. 435 



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 Volvocince for the assistance of students of this interest- 

 ing and not uncommon group of fresh-water organisms. 

 Species not as yet reported, to my knowledge, from Illinois 

 are indicated by an asterisk when found elsewhere in this 

 continent, 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 n. g. 



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 pyrenoid. 

 Asexual reproduction by repeated divisions of all of the cells, 

 each forming a daughter colony. 



P. caudata n. sp. 



Colony flattened, horse-shoe shaped, twisted about one 

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

 arranged in a marginal row of 10 or 12 and a central area 

 of 6 or 20 ; posterior end with 3 or 5 prolongations or tails 

 formed by extension of the common outer sheath. 



Known habitat, lakes and streams in central Illinois. 

 Types in the collections of the Illinois State Laboratory of 

 Natural History and deposited in the United States National 

 Museum. 



