Arcella artocrea Leidy, 1876
Diagnosis: Height of test one-fourth to one-half the diameter; dome convex; surface mammillated or pitted; border of test pronounced and rising one-fourth to one-half the height of test. Test with a distinct border. Shell brown, circular with conical aboral region which is often covered by depressions. The basal collar is usually prominent and smooth. The aperture is invaginated, circular, has a small lip and is surrounded by about thirty large and some small pores.
Dimensions: Literature 152-216 µm. My measurements: 160-202 µm.
Ecology: Bogs and Sphagnum ponds.
Remarks: This species was described by Leidy (1876). It resembles slightly Arcella vulgaris, but it differs in having pores around the aperture and a less rounded rim.