199 



genus belong to the littoral fauna among vegetation, and are adven- 

 titious in the plankton of open water. 



Euchlanis pyriformis Gosse. Recorded April 12, 1898, at 52. 

 Hempel ('99) reports it from June to October in collections in the 

 river in 1894 and 1895. 



Euchlanis triquetra Ehrbg. Average number, 19. Found irregu- 

 larly from July to November at 84 to 41. Hempel ( '99) reports it 

 also in June. It is probably adventitious. 



Hempel ('99) also reports E. dilatata Ehrbg. in, the river from 

 July to September, and E. deflexa Gosse in tributary waters. 



Gastropus stylifer Imhof. A rotifer doubtfully referred to this 

 species was found sporadically in the plankton of the river. It was 

 recorded in June, 1894, and July, 1896, at temperatures above 75. 

 It was almost continuously present in 1896 from February 20 to 

 April 10, and again on November 17 and December 3. It did not 

 reappear until January 31, 1899, from which time it continued 

 present until the close of operations in March. Most of these oc- 

 currences are at minimum temperatures and all of them below 45. 

 I have followed Weber ('98) and Jennings ('00) in using Imhof 's 

 name Gastropus stylifer instead of Hudsonella picta Zach. or Notops 

 pygmcEUs Caiman, by which names the species has been frequently 

 designated. The evidence from our records indicates that it is a 

 somewhat sporadic winter planktont in our waters. Lauterborn 

 ('93) finds it to be a perennial planktont in the Rhine, with its 

 largest numbers in summer. 



Hydatina senta Ehrbg. was found September 20 at 73. Hempel 

 ('99) also reports it in towings from the river in March and July, 

 1895. This species is very common in European waters, but has 

 as yet been found in America only in the Illinois River and, by Kel- 

 licott ('88), at Corunna, Mich. 



Mastigocerca bicornis Ehrbg. Average number, 42. Found 

 irregularly and in small numbers from June 28 to September 13 

 above 63. Hempel ('99) reports it from Quiver Lake among 

 vegetation, and it is evidently adventitious in the river plankton. 



Mastigocerca bicristata Gosse was found but once, late in Septem- 

 ber, 1895, at 73, but it is more abundant in the backwaters. 



Mastigocerca carinata Ehrbg. Average number, 1,674. This 

 species was present in the plankton from the middle of June till the 



