548 Dr. C. A. Kofoid on Platydorina. 



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 6. The cells fall into four some- 

 what irregular transverse rows, and there are the same 

 number of longitudinal ones of 4 cells each. The horseshoe- 

 shape, however, masks somewhat this simple Gon ium-Yike 

 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 (cf. fig. 1). The cells of the 

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

 obliquely outward, the direction alternating, however, in 

 conformity with the arrangement of cells in the central area, 

 as may be seen in a view of the edge of the colony (fig. 3). 

 The alternation of the cells in the colony as a whole is the 

 same whichever face is presented, the right-hand cell of the 

 posterior row of four cells always presenting the basal end 

 uppermost. From this as a starting-point the regular alter- 

 nation of stigma and pyrenoid can be traced from cell to cell 

 throughout the whole colony. An examination of twenty-five 

 colonies showed that all conformed to the same plan of alter- 

 nation, there being no case of reversal. In the arrangement 

 of the cells in the colony Platydorina is thus more like 

 Eudorina than like Gonium, being not a simple plate like the 

 latter genus, but, in reality, a flattened ellipsoid so much 

 compressed that the cells of the two faces intercalate 

 regularly, and thus give to the colony its superficial resem- 

 blance to Gonium. 



The individual cells are all substantially alike in size and 

 structure. They have the form of an oblate spheroid, slightly 

 larger in the outer hemisphere*. Some cells, especially the 



* As in the case of Eudorina and Plcodorina the terms " outer " and 



