1388 BRITISH FRESHWATER RHIZOPODA. 
The test of C. levigata is not infrequently met with 
in Sphagnum from the localities indicated, but from 
its general opacity and resemblance to full-sized in- 
dividuals of Diflugia urcula, is liable to be passed by. 
The rounded margins of the oral orifice, and the 
obliquely inverted neck, are sufficiently characteristic 
features. 
Fie. 27.—Centropyzis levigata (face and lateral views). In Sphagnum 
from Dolgoch, Merionethshire. x 260. 
Penard describes two other species of Centropyris, 
C. delicatula and C. arcelloides, both smaller than 
that under notice, and having more or less external 
resemblance to Arcellu vulgaris. They differ from C. 
levigata im the structure of the mouth and other 
features. 
Genus ARcELLA (see pp. 118-121). 
Arcella vulgaris var. compressa var. nov. (Fig. 28.) 
Arcella vulgaris (pars) Leiy Freshw. Rhiz. N. Amer. (1879), 
pl. xxvin, ff. 6, 7. 
Arcella artocrea Penarp Faune Rhiz. Leman (1902), p. 405, 
ff. (Non A. artocrea Leidy, 1879.) 
Test composed of thin chitinous membrane, yellowish 
brown and semi-transparent, becoming darker with 
age, and faintly punctated. In face view discoid, 
irregularly oval, or sub-quadrate with rounded angles, 
never truly circular; with a centrally-situated, com- 
paratively small oval mouth. In side view the crown 
