201 



should be referred. The genus is sadly in need of critical revision. 

 It includes a number of semi-limnetic species, whose importance in 

 the plankton will probably be revealed by more perfect methods of 

 collection. 



Metopidia lepadella Ehrbg. was found only in March and June 

 at temperatures above 46. It is apparently adventitious. 



Metopidia oblonga Ehrbg. was found once July 29, 1895, at 75. 



Metopidia salpina Ehrbg. w r as recorded June 28, 1898, at 78. 



Metopidia solidus Gosse. Average number, 67. This is the 

 most abundant representative of the genus in our plankton. It was 

 recorded from March 15 to November 14, at temperatures above 

 45. Most of the occurrences are in the summer months (Table I.), 

 at maximum temperatures. The numbers are small, the occurrences 

 irregular, and the species evidently adventitious. 



M. rhomboides Gosse is recorded by Hempel ( '99) from the river 

 plankton, as also M. acuminata Ehrbg., triptera Ehrbg., and bractea 

 Ehrbg. from the backwaters. 



Monostyla bulla Gosse. Average number, 50. Present in small 

 numbers and irregularly from April till the middle of October at 

 temperatures above 50. It is evidently adventitious. Jennings 

 ( '00) finds. this one of the most abundant rotifers among the aquatic 

 vegetation in Lake Erie. It is in our waters the most abundant of 

 the genus in the plankton, especially in the vegetation-rich back- 

 waters. 



Monostyla lunaris Ehrbg. Average number, 37. Found in the 

 extremes of the temperature range, but over 50 per cent, of the 

 occurrences are in August-October. Its numbers are always small 

 and its occurrences irregular. It is plainly adventitious. 



Monostyla quadridentata Ehrbg. Average number, 10. This 

 species was found in the plankton irregularly in July-September, at 

 maximum temperatures. It is abundant (Hempel, '99) in the 

 backwaters, where vegetation is abundant, and is apparently adven- 

 titious in the plankton. In addition to the species here recorded 

 Hempel ('99) lists M. cornuta Ehrbg. and M. mollis Ehrbg. from 

 collections in the river, and M. dosterocerca Schmarda from the back- 

 waters. This is an exceedingly variable group, and will repay a 

 thorough revision in the light of a study of the variation of its 

 species. A considerable reduction in the number of these so-called 

 species will doubtless result from such a study. 



