178 FRESH-WATER EHIZOPODS OF NORTH AMERICA. 



The shell, as in other named forms, occurs colorless, and of every 

 shade of brown, from the palest to the deepest. 



Viewed from above or beneath (figs. 10, 12, 14, 16), the shell resem- 

 bles a wheel with pointed cogs. Viewed from the side (figs. 11, 13, 15, 17, 

 18), it resembles a crown, sometimes with evenly rounded top and turned- 

 up rim edged with conical points, sometimes with the top depressed and a 

 series of radiating ridges curving to the points of the rim. The latter is 

 formed by an eversion of the border of the base prolonged into points of 

 variable length. Sometimes these are quite short, sometimes so long that 

 they even reach as high as the top of the shell. They vary in number, 

 usually from nine to a dozen. 



The base of the shell from a level forms an inverted concave funnel, at 

 the top of which is situated the circular, entire mouth. 



The sarcode of Arcella dentata is in all respects like that of Arcella 

 vulgaris, except that it has a more depressed or more oblate spheroid form, 

 due to the greater shallowness of the shell. 



Usually the shell of Arcella dentata is more than three times the breadth 

 of the height, and in this respect is more like A. discoides than A. vulgaris, 

 as I have distinguished them. 



The size of the shell of A. dentata presents but little variation. It 

 usually ranges from ith to ^.th of an inch in breadth between the points 

 of the rim, -Lth to Mh of an inch at the level of the base, 4,.th to i.th of 



' «i40 loO ' OlU OiU 



an inch in height, with the mouth ~th to ^,th of an inch wide, and elevated 



© ' o^'a 5*0 



from oikth to ~th of an inch. 



2500 1500 



ARCELLA ARTOCREA. 



Plate XXX, figs. 1-9. 

 Arcella artocrea. Leidy: Proc. Ac. Nat. Sc. 1876, 57. 



Shell from a fourth to less than half the height of the breadth; dome 

 convex and even, or mammillated or pitted; basal border everted and 

 rising from a fourth to nearly half the height of the shell, obtusely angular 

 and entire; central portion of the base inverted in the usual concavely 

 infundibuliform manner; mouth circular, entire, surrounded with a circle of 

 minute tubercles. Sarcode having the same general form and relationships as 

 in other Arcellas, but rendered bright green from the presence of abundance 

 of chlorophyl corpuscles in the endosarc. Pseudopods colorless, digitate 



