Specimen with longitudinal division, with the older part at the left, and the newer one with a developing collar - eutrophic ditch, floating debris - Horstermeer.
Rhizaspis scutiformis var. marginata (De Groot, 1979)
This variety was listed and depicted by De Groot without any further diagnostic description. I find it regularly, but it's not clear if this form falls within the variation range of R. scutiformis.
Impression of the longitudinal division of a specimen. a: photomicrograph; b: start of division, the new veil becomes visible; c: the cell divides and d: the cells has separated (b-c: drawings Ferry Siemensma). Below two images which show the same cells in aboral view.
The above specimen in division, viewed from above.
Test 87 µm - Gaasterland.
Arrow points to a lateral wing.