GENUS DIFFLUGIA— DIFFLUGIA CONSTEICTA. 123 



sequence of the structure of the shell obscuring the interior, to determine 

 whether the specimens under examination are dead or alive. 



Difflugia constricta, in its various forms, is one of the most abundant of 

 species, and appears to be found almost everywhere where moisture and 

 alga; are present. 



Small spineless specimens, cap-like in form, of the variety named by 

 Dr. Wallich Difflugia cassis, such as represented in figs. 8-34, pi. XVIII, 

 are very common with alga?, on the surface of moist earth, in marshy 

 places, in meadows, and in forests. I have found them constantly, in asso- 

 ciation with the common wheel-animalcule and several other rhizopods, 

 about the roots of mosses, and with algae, in the crevices of the pavements 

 in shaded places in the city of Philadelphia. I have even found them 

 among mosses and lichens high up in trees. They also occur frequently 

 among sphagnum. 



The small spineless forms are frequently of various shades of brown, 

 while the larger ones are usually colorless. 



The largest varieties and the spine-bearing forms are found in the ooze 

 of ponds. These appear to constitute the variety named by Dr. Wallich 

 Difflugia marsupiformis. See figs. 35-55. 



A somewhat peculiar variety, represented in figs. 37-44, I have not 

 unfrequently found among sphagnum. The shell is cap-like in shape, 

 yellowish brown in color, and composed of chitinoid membrane, usually 

 incorporated with variable proportions of scattered sand particles. In the 

 view from the front or back, the shell appears transversely oval, but flat 

 beneath. The mouth is deeply inflected, or forms the smaller opening of 

 an inverted funnel, of which the base of the shell forms the greater opening. 

 Sometimes the fundus of the shell is loaded with comparatively large 

 stones, and rarely the shell is almost devoid of sand particles. This form 

 of shell approximates closely, if it does not really merge into, the spineless 

 variety of Centropyxis. 



Either of the names Difflugia marsupiformis or D. cassis, given by Dr. 

 Wallich, better applies to the species than that of Arcella constricta, given 

 originally by Ehrenberg. The apparent constriction, often absent, is due to 

 the narrowing of the shell, as seen in the front view, opposite the position 

 of the mouth. 



The form of Difflugia constricta repeats that of Trinema cnchelys, but 



