432 Illinois State Laboratory of Natural History. 



100 rotations from left over to right to 355 from right over to 

 left. In this respect, then, so far as there is evidence, 

 Platydorina is the most highly differentiated genus of the 

 family. 



The polarity of the lower genera, Stephanosphcera and 

 Gonium, is likewise of the simplest form, being merely physio- 

 logical, the same pole or face of the colony always leading in 

 locomotion. In Pandorina, Eudorina, and Volvox, however, 

 there is the added feature of the greater brightness of the 

 anterior stigmata, and in Pleodorina the two poles are differ- 

 entiated by the two types of cells as well as by the characters 

 found in the genera just mentioned ; but Platydorina is the 

 only genus of the family in which polarity is expressed by the 

 arrangement of the cells and by structural features of the 

 envelope. In regard to polarity, also, the new genus is thus 

 the most highly specialized member of the Volvocince. 



The reproduction of Platydorina has been observed by me 

 repeatedly in the past five years, but only the asexual phase 

 has thus far been discovered. All of the cells of the organism 

 are gonidial, each dividing to form a daughter colony. The 

 sequence of the divisions and the position of the successive 

 planes are of the type found in Eudorina and Pleodorina, 

 the resemblance being so close that the figures illustrating the 

 asexual development of Pleodorina illinoisensis ('98, 

 PI. XXXVII.) might almost be used for cleavage in Platy- 

 dorina. There is one difference, however^ for in Platydorina 

 the curved plate of cells, which becomes first cup-shaped and 

 then ellipsoidal, subsequently flattens, the cells of the two 

 faces intercalating during the process. The daughter colony 

 acquires the adult form, including the tails and the torsion of 

 the plate, before it escapes from the maternal matrix, the 

 young colonies moving about for some time in the disinte- 

 grating matrix before making their escape through the ruptured 

 outer sheath. The secondary sheath surrounding the gonidial 

 cell becomes the outer or primary sheath of the new colony. 

 No stages of sexual reproduction have been seen, though the 

 collections examined represent a considerable range of season 

 and locality. It may be that these are to be sought upon 



