556 Dr. C. A. Kofoid on Platydorina. 



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. 



Keys to the Genera and Species of the Subfamily Volvocinaj. 



Cells arranged in coenobia of definite forms varying with the species, 

 biflagellate, with stigma and one or more chromatophores ; surrounded 

 by a gelatinous envelope, whose development separates them to a greater 

 or less degree ; number not uniform, varying in the different species, 

 often, but not always, definite. Asexual reproduction by repeated divi- 

 sions of gonidial cells, which constitute the whole or only a part of the 

 parental organism, to form daughter organisms ; sexual reproduction in 

 some species (in others unknown) by the conjugation of male and female 

 gametes, resulting in the formation of a resting stage, which later develops 

 into a new organism. 



J 



Genera. 



Cells arranged in form of plate, with flagella upon 



one face only 2. 



Cells arranged in spherical, ellipsoidal, or flat- 

 tened colonies ; flagella not confined to one 



t face 3. 



f Cells in a squarish plate; envelope closely ad- 



2 \ herent Gonium. 



"j Cells in a rounded plate ; envelope swollen, oval, 



[ or spherical Stephanosphtera. 



( Colony ellipsoidal or spherical ; cells crowded 

 together, conical, reaching towards centre, 



3-{ outer membrane of concentric layers Pandon'na. 



j Cells not crowded together nor reaching towards 



centre of colony 4. 



f Colonies ellipsoidal or flattened; cells uniform in 



size 5. 



4^ Colonies spherical or spheroidal, or, if ellipsoidal, 

 with small vegetative and large gonidial 



{ cells 6. 



f Colony ellipsoidal or spherical; poles not differen- 

 tiated by arrangement or size of cells or by 



,. ; structure of envelope Eudorina. 



) Colony flattened, horseshoe-shaped, with poles 

 differentiated by arrangement of cells ; poste- 

 rior end with tails Platydorina. 



I Cells not connected by protoplasmic processes, of 

 two sizes, smaller vegetative at anterior pole 



(j ^ and larger gonidial at posterior Pleodorina. 



| Cells connected by protoplasmic processes, not 



markedly different in size Volvox. 



