Phylum: Charophyta   Family: Desmidiaceae

Xanthidium smithii  W. Archer 1860

The length of spines varies considerably but would seem to be between populations rather than within. West & West (1912: 61, pl. 111: 10) reproduce a drawing of a zygospore after Roy & Bisset (1894, pl. 2: 20). They described it as globose with long, simple, acute spines with a widely conical base. For dimensions they give 25-26 µm without spines, 66-68 µm with spines. The Outer Hebridean finds agrees with the first measurement but has substantially shorter spines with an inflated base.
Cell dimensions: L: 26-30 µm; B: 24-30 µm; Is: 8.7-10.7 µm; Sp: 5-13 µm; overall LxB: 38-47 x 39-48.
Zygospore dimensions: minus projections: 26 µm; overall: 44 µm.
West & West (L.c.) note it as very rare even in their day with the main concentrations in North Wales, the west of Ireland and the Outer Hebrides. The preferred habitat here is acidic bogs and moorland pools, where it is still relatively common.

References: 
Coesel, P.F.M. & Meesters, K.J. (2023 second edition). Desmids of the Lowlands.
Roy, J. & Bisset, J.P. (1894). On Scottish Desmidieae.
West, W. & West, G.S. (1912). A Monograph of the British Desmidiaceae, Volume 4.