129 



Strombidium viride Stein was found in small numbers in January- 

 March, 1899, at minimum temperatures. 



Stylonychia mytilus (O. F. Mull.) Ehrbg. was found in the plankton 

 sparingly from September to February, and once in June. 



Tintinnidium fluviatile Stein. Average number, 22,590 or 1,640,- 

 192*. This species is somewhat sharply limited to the warmer 

 months in its seasonal distribution. In 1898 (Table I.) it makes its 

 appearance April 4 at 49, reaches a maximum of 720,000 May 3 at 

 60, and has three decreasing pulses ; one of 1 04, 000^ oa June 14 at 80, 

 one of 95,200 on August 2 at 79, and one of 22,400 on September 27 

 at 73, and disappears from the plankton October 18 at 52. The 

 records in previous years are more irregular, though traces of vernal 

 and midsummer pulses can be found in the records. Filter-paper 

 catches indicate that only one in eighty of this species is retained by 

 the silk. They also locate the pulses as approximately coincident 

 with those of the silk collections. 



Apstein ('96) finds Tintinnidium to be a spring planktont with 

 its maximum in April in Lake Plon, while Seligo ('00) finds it in lakes 

 near Danzig in the autumn, with a maximum in September. In our 

 own waters in 1896 the autumnal pulse in August-September exceeds 

 the vernal one. 



The gelatinous lorica of this species is subject to great variation in 

 its size and proportions, and especially in the region about the aper- 

 ture. A somewhat thimble-shaped form was described by Hempel 

 ('96) as T.illinoisensis, the specific distinctions being based wholly on 

 the lorica. This form intergrades with the typical lorica of T. 

 fluviatile Stein, and should not in my opinion be given specific rank. 



Trachelius ovum Ehrbg. Average number in 1895, 847. This 

 species did not occur in 1898 but was rather common in November- 

 December, 1895, reaching a maximum of 10,695 on December 4 at 

 32.5. Isolated appearances in small numbers in December and 

 January of other years have been recorded. In our waters it is thus 

 a winter planktont. Stagnation conditions under the ice were 

 approaching (Pt. I., PI. XLIII.) when the pulse of 1895 occurred in 

 the Illinois River. Apstein ('96) found it, however, in Lake Plon with 

 a maximum in May- June, disappearing in the summer and returning 

 again in November. 



Trichodina pediculus Ehrbg. Average number, 1; in 1897, 874. 

 This species is normally found upon Hydra, on the gills and skin of 



