No. 2.] FRESH-WATER PROTOZOA. I5 



frequently arising after the withdrawal of the ray-like 

 pseudopodia. Nuclei and contractile vacuoles always 

 present. 

 Dae. radiosum Ehrbg., Fig. 9. 

 Arcella Ehrbg. 



Shell yellow to dark brown, watch-glass shape, curved 

 or polyhedral. The side bearing the psudopodia flat, with 

 central opening. The shell shows a fine hexagonal lattice- 

 work, due to its being made of hexagonal prisms. Body 

 substance does not completely fill the shell. Nuclei and 

 vacuoles usually numerous. Reproduction by fission and 

 budding. 

 Arc. dentata Ehrbg., Figs. 10 and loa. 

 Arc. vulgaris Ehrbg., Fig. ii. 

 Centropyxis Stein. 



Like Arcella, but armed with a variable number of 

 spines. This genus is closely related to Arcella, and is 

 commonly regarded as the same. 

 Cent, aculcata Stein, Fig. 13. 

 Quadrula F. E. Sch. 



Shell pear-shaped, slightly compressed ; composed of 

 quadrilateral (silicious?) plates. Posterior end sometimes 

 armed with spines. 

 Q. symmetrica Schul., Fig. 20. 

 Difflugia Leclerc. 



Shell formed of foreign bodies, grains of said, diatom 

 shells, etc., united by a chitinous connecting substance. 

 Form spherical to pear-shaped; frequently with spines at 

 the posterior end. Mouth occasionally excentric. Body 

 not completely filling the shell ; with finger-like pseudo- 

 podia. Nucleus and contractile vacuoles variable. 

 Dif. globosa Duj., Fig. 14. 

 Dif. lob stoma Leidy, Fig. 16. 

 Dif. pyriformis Perty, Fig. 17. 

 Dif. cratera Leidy, Fig. 19. 

 Dif. acuminata Ehrbg., Fig. 22. 

 Dif. corona Wall., Fig. 27. 

 Lecquereusia Schlumbg. 



Shell of sand or specially formed bodies. Shell spirally 

 coiled, laterally compressed, but the neck cylindrical. 



