274 Illinois State Laboratory of Natural History. 



condition. Volvox, Euglena, Phacus, Lepocinclis, Traehelo- 

 monas, Dinobryon, Synura, Mallomonas, Uroglena, Melosira, 

 and Fragillaria occurred in varying frequency, but only a single 

 specimen of Pleodorina californica was found in collections 

 containing the species described in this paper. The animal 

 plankton was represented in the main by rotifers, Polyarthra 

 being most abundant, while Synchceta, Euchlanis, Pterodina, 

 Brachionns, and Anurcea were also present. Difflugia, 

 Codonella, Bosmina, Cyclops and nauplii complete the list 

 of the more common associates of this Pleodorina in the 

 plankton. 



Pleodorina illindisensis n. sp. 



The species here described consists of an ellipsoidal eoeno- 

 bium or colony of 32, rarely 16 and still more rarely 64, 

 biflageUate cells. The shape is quite constant, occurring in 

 the youngest colony and continuing throughout the asexual 

 cycle until the daughter colonies abandon the gelatinous 

 matrix of the maternal organism. Among the large number 

 examined only a few specimens were seen which approached 

 a spherical form. Measurements of tw r elve seemingly full- 

 grown colonies from material freshly killed in 2% formalin 

 showed a range of 101 to 137 /< in long diameter, and an 

 average of 113 j(. The transverse diameter ranged from 84 

 to 102 f-i, and averaged 94 /<. Individuals in which the 

 gonidia have begun to divide show a considerable swelling of 

 the hyaline gelatinous envelope. One specimen containing 

 2- and 4-cell stages measured 178x155 //, and when 

 the young colonies are ready to escape, the parent may meas- 

 ure as much as 200x175 )t. At the time of escape the 

 young colonies measure 46x38 /'. The measurements of 

 the colonies approach very closely those given by Butschli 

 ('80-89, p. 840) for Eudorind; viz., 100-150 n ; and the 

 colonies of this genus found in association with the form here 

 described exhibit dimensions almost, if not quite, identical 

 with those above recorded for the Pleodorina. 



The colony (PL XXXVI., Fig. 1) contains, as a rule, 32 

 cells arranged, as Henfrey \'5i\) first noted for Endorina, in 



