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. was 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 
_(700) 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.closterocerca 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. 
