222 



FRESH-WATER BIOLOGY 



40 (41) Shell strongly convex. 



41 (40) 



42 (39) 



. . . Arcella vulgaris Ehrcnberg 1830. 



Shell may be smooth or with re^'ular undu- 

 lations. Protoplasm united to the inside of the 

 shell by delicate threads. Pseudopodia long, 

 straight and very transparent. Contractile 

 vacuoles numerous. Nuclei two, opposite in 

 position. This species shows great variation in 

 size and form. \'ery common in pond water. 

 Diameter 80 to 140 n. 

 Fig. 289. Arcdlavul\:,aris. Lateral and interior views 



oi the same individual. X 150. (After Leidy.) 



Shell very flat Arcella discoides Ehrcnberg 1843. 



Shell smooth, regularly punctate, with a large circular aperture. It 

 is a shy species, the pseudopodia seldom being observed. Contractile 

 vacuoles numerous. Nuclei two. Common in pond water. Diameter 

 from 72 to 264 /i. 



Fig. 290. Arcella d'ncoide^. X i75- (After Penard.) 



Shell periphery with projecting points. 



Arcella dentata Ehrcnberg 1830. 

 When viewed laterally the shell has the appearance of a crown, 

 the teeth-like points being produced from the base of the low 

 dome. Nuclei two; contractile vacuoles numerous. Habitat 

 bogs and swamps. A rare species. Diameter 132 to 184 m- 

 Fig. 291. Arcella dentata. Lateral and inferior views of the same indi- 

 vidual. X 100. (After Leidy.) 



43 (38) Shell with punctae sometimes indistinct. Aperture very wide. 



Pyxidicula Ehrcnberg. 

 Representative species Pyxidicula cymhalum Penard 1902. 



Shell pateUiform, brown in color, with distinct punctae. Aper- 

 ture round, nearly as wide as the diameter of the shell, bordered 

 by a narrow rim. Contractile vacuole single. Nuclei probably 

 two. Pseudopodia not observed in this species. Identified by 

 Penard in material from Summit Lake, Colorado. The only spe- 

 cies of the genus thus far reported from North America. Diameter 

 85 to 90 M- Habitat mosses. 



44 (37) Shell without regular markings, but with foreign particles attached. 



Aperture eccentric Centropyxis Stein. 



Representative species Centropyxis aculeata Stein 1857. 



Shell compressed laterally, resulting in both mouth and fundus 

 being eccentric. Color some shade of brown. Slender spines 

 usually developed from the fundus. Nucleus single; contractile 

 vacuoles two or more. The species is very shy, sometimes ex- 

 tending a single large pseudopodium. A common species among 

 algae. Diameter 88 to 260 /i- 



Fig. 293. Centropyxis aculeata. X 150. (After Leidy.) 



45 (36) Shell not discoidal 46 



46 (51) Shell spiral, compressed, largely composed of minute, curved, rod- 



like plates Lecquereusia Schlumberger . . 47 



47 (48) Shell primarily of sand grains, few plates. 

 ^3^ Lecquereusia modesta Rhumbler 1845. 



This species has a short, broad neck, slightly turned to one side. 

 Nucleus single. Pseudopodia few, large and long. Found among 

 mosses in swamps. Length from 125 to 150 m- 



Fig. 294. Lecquereusia modesta. X 125. (After Penard.) 



Fig. 292. Pyxidicula cym- 

 balum. X 210. (After 

 Penard.) 



