141 



when the vegetation was common along the margins of the stream, 

 it was taken in the plankton occasionally. 



Megalotrocha spinosa Thorpe. Isolated individuals of this un- 

 usual species were taken in small numbers in the plankton in August, 

 1896, at maximum temperatures, but no colonies were observed. 

 This is one of the largest of the rotifers in the plankton, individuals 

 measuring about 1 mm. in length. The species was described by 

 Thorpe ('93) from Chinese waters; was next reported by Weber ('98) 

 from the neighborhood of Geneva, Switzerland; and Jts_ occurrence in 

 the Illinois is, I believe, the third record of its appearance. It 

 affords another illustration of the cosmopolitan nature of the fresh- 

 water plankton. In both Chinese and Swiss waters it was associated 

 with M. semibullata Thorpe, also from Hong Kong and Brisbane. 

 This latter species occurs in our waters also (Hempel, '99), though it 

 was not taken in the plankton with M. spinosa. 



BDELLOIDA. 



Average number, 7,807. They were less numerous in 1897, a 

 year of more stable hydrograph, and fully twice as abundant in 1896, 

 when river levels were much disturbed during summer months. In 

 their seasonal distribution, save for the increase of Rotifer tardus in 

 the winter of 1898, the bdelloid rotifers reach their greater numbers 

 in the plankton in the period from March to November. There is a 

 trace of a vernal pulse in April-May (Table I.), and some irregular 

 summer fluctuations, attributable in the main to floods. Their tem- 

 perature optimum seems (except in the case of R. tardus above noted) 

 to lie above 50. They are as a rule adventitious in the plankton, 

 owing their presence in some cases to floods, though the vernal in- 

 crease can not in most cases be attributed directly to this disturbance. 

 The species are difficult to identify in preserved plankton material, 

 and the list here cataloged is small. Examination of living plankton 

 would considerably extend the list of forms,. 



DISCUSSION OF SPECIES OF BDELLOIDA. 



Philodina citrina Ehrbg. was found in the plankton but once 

 September 14, 1897. 



Philodina megalotrocha Ehrbg. Average number of females, 351. 

 This species was found in the plankton (Table I.) from March 15, at 

 46, to November 8, at 45. The distribution in previous years fell 



