426 Illinois State Laboratory of Natural History. 



The arrangement of the cells is characteristic, and is 

 strikingly different from that of any other genus of the family. 

 The gelatinous matrix and sheath conform to the horse-shoe- 

 shaped plate of cells, and even the caudal appendages bear a 

 fixed relation to the plan of cell arrangement. The 32-cell 

 colony is composed of a marginal U-shaped row of 12 cells 

 about three sides of a 20-celled somewhat rectangular plate, 

 which, in turn, consists of an outer row of 12 cells on three 

 sides of a row of four pairs of cells. The colony might also 

 be regarded as made up of three U-shaped rows of 12,12, and 

 8 cells respectively, nested in such a fashion that the inner 

 two project one cell beyond the outermost. The cells also 

 fall into six quite irregular transverse rows of 4, 6, 6, 6, 6, 

 and 4 cells respectively, and into the same number of corre- 

 sponding longitudinal ones. As before stated, the lateral 

 tails are posterior to the marginal row, while the postero- 

 laterals are behind the first row within the marginal, and the 

 median one midway between the innermost rows. In the 

 colony of 16 cells (Fig. 2) the marginal row has but 10 cells 

 and the central plate but six. The cells fall into four some- 

 what irregular transverse rows, and there are the same 

 number of longitudinal ones of 4 cells each. The horse-shoe 

 shape, however, masks somewhat this simple Gonium-like 

 arrangement. The plate-like form of the colony and the 

 arrangement of the cells, especially in the 16-cell form, give 

 this new genus a superficial resemblance to Gonium. It is, 

 however, fundamentally different, for in Platydorina the two 

 faces of the plate are exactly alike, while in Gonium the face 

 anterior in locomotion bears all the flagella, and the other 

 face presents only the bases of the cells. This similarity of 

 the two faces in Platydorina, neither of which is anterior or 

 posterior, is brought about by the fact that every other cell 

 upon either face presents to that face the pole which bears 

 the stigma and the flagella, while the intervening cells present 

 the opposite pole, with its pyrenoid. This alternation of stigma 

 and pyrenoid is constant, and can be followed in any row of 

 cells except the diagonal ones (c/., Fig. 1). The cells of the 

 marginal row, in both the 16- and 32-cell colonies, point 



