Phylum: Charophyta   Family: Desmidiaceae

Cosmarium venustum var. hypohexagonum  West

This interesting find with a zygospore fits this variety based on current knowledge and references. Kouwets (pers. comm.) who is working on a major revision on the genus has many misgivings about the authenticity of some of the reports. It's quite possible that this will be subsumed into the nominate variety as natural variation. The species is known for considerable variation

References: 
West, W. & West, G.S. (1908) A Monograph of the British Desmidiaceae, Volume 3