Phylum: Charophyta   Family: Desmidiaceae

Cosmarium rostafinskii R. Gutwinski 1890

This species was made a variety of C. speciosum by West & West (1908: 251) with the comment that “… it differs so little from C. speciosum that it is scarcely justifiable to regard it as specifically distinct.” More recent finds in the Netherlands by Van Geest & Coesel (2019) have shown the cells to be more pyramidal that C. speciosum and with a variant granular pattern as shown with SEM imaging. This prompted them to reinstate it as a separate species. The semicell sides are slightly convex and run smoothly into the slightly domed apex. There are about 20 undulations visible in face view.
Cell dimensions: L: 43.8-49.9 µm; B: 28-29.7 µm; Is: 15.6-16.9 µm; Th: 20.5-22; L/B: 1.53-1.68.
A rare desmid found in atmophytic habitats such as ephemeral pools and with moss on wet rocks. The only samples from the Outer Hebrides have come from a single site: a granite depression subject to desiccation.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium. Part 1: Text. Part 2: Plates.
Van Geest, A. & Coesel, P.F.M. (2019). Some new and interesting desmids (Streptophyta, Desmidiales) from ephemeral puddles in the urban and industrial areas of Amsterdam (Netherlands).