Phylum: Charophyta Family: Desmidiaceae
Cosmarium regnesi var. tritum West
Differs mainly from the nominate variety by have a 'V-shaped' sinus. The habitat is shallow moorland pools and bogs which are slightly acidic.
References:
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3