544 Dr. C. A. Kofoid on Platydorina. 



and Rotifer, Philodina, Asplanchna, Euchlanis, Cathypna, 

 Pistyla, Monostyla, and Pterodina were often represented by 

 one or more species. The paucity of Entomostraca stands in 

 sharp contrast with the abundance of rotifers, the former 

 group being represented by relatively few species and few 

 adult individuals. The nauplii of Cyclops were, however, as 

 a rule, abundant, but only occasional specimens of adult 

 Cyclops, Diaptomus, Bosmina, Chydorus, Ceriodaphnia, 

 Dophnia, and Cypridopsis were to be seen. 



Platydorina caudata, gen. et sp. n. 



The species here described consists of a horseshoe-shaped 

 ccenobium or colony (PI. VII. fig. 1) of 16 or 32 biflagellate 

 cells, the anterior end corresponding to the toe of the horse- 

 shoe and the truncate posterior end to the heel, the latter 

 carrying 3 or 5 prolongations or tails formed by the gelatinous 

 substance of the ccenobium. The colony is plate-like and 

 flat, except that the plate is slightly twisted in a left spiral. 

 This spiral is scarcely noticeable in a face view (fig. 1) except 

 by focussing with a high-power objective, but it can be easily 

 detected in a side view (fig. 3). It varies from one eighth to 

 one thirty-second of a turn of the spiral, and in twenty-five 

 colonies especially examined on this point it was invariably a 

 left spiral, with the location of the twisting always in a defi- 

 nite relation to the colony, the right anterior and the left 

 posterior regions of the colony in face view being high, while 

 the left anterior and the right posterior are low. Repeated 

 examinations of specimens, both living and preserved, indi- 

 cate that this spiral form is a constant feature of structure, 

 that it is not reversed in direction, and that it is subject to 

 but slight variation in the degree of the torsion. No move- 

 ment within the colony which would produce or vary the spiral 

 was noted in living individuals. The form of Platydorina 

 seems to be as constant and as characteristic as that of other 

 genera of the family. 



The size of the colony varies with the age, with the number 

 of cells present, and also perhaps with the locality and the 

 season. Colonies of 32 cells in which the first division 

 leading to the foimation of daughter colonies is taking place 

 average about 150 p in length, 130 p in width, and 20 p in 

 thickness. The largest colonies are about 165 X 145 X 25 p, 

 and the smallest free-swimming ones about 25x21x4 p. 

 Colonies of 16 cells are smaller than those of 32 cells and 

 are also narrower in proportion to their length, measuring 

 about 70 x 43 x 16 p. 



