180 FRESH-WATER RHIZOPODS OF NORTH AMERICA. 



0ENTR0PYXI8. 



Greek, kentron, a prickle ; piacis, a box. 



Arcella: Ehrenberg. Difflugia : Perty. Centropyxia: Stein, 1857. 



Eohinopyxie: Claparfede and LachmaTiii, 1859. Homcenchlamya : Ehrenberg, 1871. 



Shell discoid, circular, oval, or ovoid, deepest or thickest and most 

 obtuse posteriorly, with the base on a level inferiorly and deeply inflected 

 to the mouth, with the dome highest posteriorly, gently curving or sloping 

 forward, more abruptly convex backward; mouth and fundus eccentric 

 in opposite directions, the former anterior and inferior, the latter posterior 

 and even, or furnished with a variable number of conical spines. Mouth 

 circular or oval and entire, or with the border more or less deeply sinuous, 

 and extending into appendages within the shell toward the dome. Shell 

 usually of various shades of brown, but sometimes colorless, composed of 

 Chitinoid membrane mostly incorporated with variable proportions of 

 quartz-sand, and often to such an extent as to assume the structure common 

 in Difflugia. Sarcode colorless; pseudopods digitate. 



Centropyxis, according to my experience, always appears distinctly 

 separated from Arcella, while, on the other hand, it appears more closely 

 related with Difflugia through D. constricta. The shell of- Centropyxis, as 

 in the lattei*, has the mouth and fundus eccentric in opposite directions, 

 while in Arcella they are both central in the same longitudinal axis. The 

 shell of Arcella is chitinoid, with a distinct hexagonal cancellated structure, 

 and is always free from extraneous particles, whereas in Centropyxis the 

 shell, though often membranous or chitinoid, does not exhibit a cancellous 

 structure, and is mostly incorporated with more or less extraneous matters. 



CENTROPYXIS ACULEATA. 



Plates XXX, figs. 20-34; XXXI; XXXII, figs. 29-37. 



Arcella aciflmta. Ehrenberg : Ab. Ak. Wis. Berlin, 1830, 40 ; 1841 , 368, Taf. iii. Fig. 5. Infusionsthierchen, 



1838, 133, Taf. ix. Fig. vi. 

 Arcella ecarnia. Ehrenberg : Ab. Ak. Wis. Berlin, 1841, 368, Taf. i. Fig. 9 ; Taf. iii, Fig. 46. Microgeologie, 



1854, Taf. xxxiv, ii, Fig. 1. 

 Difflugia aculeata. Perty : Kennt. kleinst. Lebensformen, 1852, 186. 

 Centropyxia aculeata. Stein: Sitz. Buhm. Gesells. Wis. ? 1857. 

 Mchinopyxia aculeata. Claparfede and Lachmann: iStudes Infus. 1859, 447.— Carter: An. Mug. Nat. Hist. 



xiii, 1864, 29, pi. i, fig. 8.— Barnard : Proc. Am. As. Adv. So. xxiv, 1875, 241. Am. Quart. 



Mio. Jour. 1879, 83, pi. viii, fig. 3. 

 A. Centropyxii aculeata. Ehrenberg: Ab. Ak. Wis. Berlin, 1871, 245. 

 A. Homceochlamya ecornw. Ehrenberg : Ibidem, 244. 

 A. Centropyxia Diadema. Ehrenberg: Ibidem, 245. 

 Arcella Diadema. Ehrenberg : Ibidem, 259, Taf. iii. Fig. 7, 8. 

 Centropyxia. Leidy: Pr. Ac. Nat. Sc. 1876, 57. 

 Centropyxia ecarnia, variety. Leidy, 



