318 Illinois State Laboratory of Natural History. 

 Family ARCEL.LID.ffi. 



ARCELLA EhkbG. 



Shell cbitinous, usually round, convex above, concave be- 

 neath with a large round opening at middle ; color yellow or 

 brown, its surface smooth or thickly pitted ; usually several 

 nuclei and contractile vacuoles. 



4. A. vulgaris Ehrbg, 



This species occurred in small numbers in the towings 

 throughout the year, except during the winter months, and in 

 summer was occasionally found in collections from the vegeta- 

 tion along shore. It seems remarkable that none were found 

 in towings from Thompson's Lake except a few which 

 appeared in March, 1895. 



5. A. vulgaris discoides Leidy. 



This variety was usually found in company with the typ- 

 ical form, in about the same numbers and similarly distri- 

 buted. A single finding in midwinter at C is recorded, also 

 its occurrence in Matanzas Lake. 



6. A. vulgaris angulosa Leidy. 



Not so common as the preceding forms, but similarly dis- 

 tributed. 



7. A. dentata Ehrbg. 



This form was very rare, and was seen but twice ; once dur- 

 ing August, and again in September, in towings from sub- 

 station C. 



DlFFLUGIA LecleRC. 



Shell variously shaped, sometimes cbitinous, but usually 

 composed principally of small grains of sand and other for- 

 eign substances. Pseudopodia narrow, long, sometimes 

 branching, rounded at the ends. Nucleus usually single. 



8. D. globulosa Duj. 



This was widely distributed, occurring at all the substa- 

 tions, usually in the towings, and much less frequently in the 

 Birge-net collections. It was never very common, but con- 



