Phylum: Charophyta   Family: Desmidiaceae

Cosmarium corriense J.P. Bisset in J. Roy 1894

A medium sized cell, about 1.4 times longer than broad, sinus open and parallel for half its length before opening widely, with a slightly oblique apex. Semicells are sub-rectangular with almost straight sides and broadly rounded upper angles with a convex apex. Cell-wall ornamented with a regular vertical series of small granules; the basal granules are doubled; apical view broadly oval, lateral view subcircular. Chloroplast axile with one pyrenoid. A very rare desmid first found by J.P. Bisset in a quarry at Corrie, Isle of Arran, Scotland; see in Roy (1894: 44, pl. 2: 6). It was named after the site. Interestingly, the drawing shows a uniform series of single granules with no mention of the row nearest the isthmus being doubled. The measurements given: L: 43 µm; B: 32 µm; Is: 15 µm suggest just a single cell was found. West & West (1908: 133) refer to Bisset’s find and give a thickness of 21 µm (presumably measured from Bisset’s drawing). They also note another find in Kerry, Ireland. It should be compared with the superficially similar Cosmarium porteanum W. Archer 1860. That species has rounded laterals, a more extended isthmus, no doubled granules and the size is smaller with little overlap.
Cell dimensions: L: 40.1-47.1 µm, B: 30.1-33.9 µm, Is: 14.5-16.2 µm, Th: 23.0-24.9 µm. L/B: 1.31-1.41.
A rare desmid that has been found several times in the Outer Hebrides, all from sub-atmophytic habitats, mainly with moss on dripping rocks. A population from Harris was abundant.

References: 
Kouwets, F.A.C. (2025). European Flora of the Desmid Genus Cosmarium.
Roy, J. (1894). On Scottish Desmidieae.
West, W. & West, G.S. (1908). A Monograph of the British Desmidiaceae, Volume 3.