GENUS CENTKOPYXIS— CENTEOPTXIS ACDLEATA. 181 



Centropijxis aculeata (pi. XXX, figs. 20-34; pi. XXXI; pi. XXXII, 

 figs. 29-37) is one of the most common of the Lobose Protoplasts, and is 

 found everywhere in the usual localities of Arcella vulgaris It is exceed- 

 ingly variable in character, but I have not been able to distinguish more 

 than the one species. This is to be sure not always aculeate, as expressed 

 by the name; but the spineless form is evidently a mere variety. 



The spinous forms may be regarded as the more characteristic, as 

 represented in the figures of pi. XXXI, except the last two of the series, 

 and as seen also in figs. 29-34 and 37 of pi. XXXII. The spineless forms, 

 constituting the Arcella ecornis of Ehrenberg, represented in figs. 20-34, pi. 

 XXX, figs. 33, 34, pi. XXXI, and figs. 35, 36, pi. XXXII, may be regarded 

 as a variety with the name of Centropijxis ecornis. 



Size. — Length of shell transversely from 0.088 mm. to 0.26 mm.; 

 breadth 0.072 mm. to 0.22 mm. ; height 0.036 mm. to 0.08 mm. ; diameter 

 of mouth 0.028 mm. to 1 mm ; length of spines 0.02 mm. to 0.06 mm. 



Locality. — Everywhere in association with Arcella vulgaris and quite as 

 common. Abundant in ditches and ponds in Pennsylvania, New Jersey, 

 Rhode Island, Massachusetts, Florida, Louisiana, Colorado, Wyoming 

 Territory, Utah, and Nova Scotia. 



The shell of Centropyxis aculeata as commonly observed reminds 

 one of an Arcella with the fundus pressed to one side, so as to render the 

 mouth eccentric. As usually seen beneath the microscope, from above or 

 below, it appears with a more or less broadly ovoid outline, with the 

 mouth nearer the narrower pole and a variable number of spines diverging 

 from the opposite pole ?nd sides. 



In the lateral view, the shell is cap-shaped, like the shell of Difflugia con- 

 stricta, but commonly more depressed. The mouth and fundus being eccen- 

 tric in opposite directions, the former is anterior and inferior, and the latter 

 posterior and more elevated than the fore part of the shell. The greatest 

 perpendicular depth of the shell is back of its middle, and its shallowest 

 portion forms the anterior border, which often is somewhat depressed below 

 the general curvature of the front of the dome. The base of the shell 

 rests on a level at its anterior two-thirds, and is inverted funnel-like as in 

 Arcella. The fundus of the shell is obtusely rounded, and is usually 

 furnished with a variable number of divergent spines arranged in a single, 



