Difflugia mammillaris Penard, 1893
Diagnosis: Test ovoid, elongate, swollen or arched in the mid-region but tapering at both extremities, to give a rounded protuberance aborally and gradually near the aperture to give a slightly pronounced neck. Aperture circular, composed of small particles; in side view usually oblique. Aboral spine usually non-central. Nucleus spherical with central nucleolus. Nucleolus with some small lacunae.
Dimensions: Length 93-111 µm (Ogden), 90-130 µm (Penard) and 90-113 µm (Grospietsch). My measurements:97-129 µm.
Ecology: Freshwater; different water types. Common species in sediments and detritus; when present, it is abundant.
Remarks: The test and its extremity can strongly vary, but nevertheless this is a distinct species, easy to recognize when several shells within a population can be observed. Though Ogden describes the test as colorless, my specimens were always light brown or dark grey.
This species resembles Difflugia elegans, but that species doesn’t have a rounded protuberance. It can also be confused with Difflugia varians.