Galanthus angustifolius is a rare species from the northern Caucasus allied to Galanthus alpinus.
It has narrow glaucous leaves and large flowers.
The name has also been applied to a narrow leaved form of Galanthus nivalis.
I bought my bulb from Broadleigh Gardens in 1987, and it has slowly clumped up.
Bishop, M., A.Davis and J.Grimshaw. Snowdrops, A monograph on cultivated Galanthus. Griffin Press Publishing ltd. 2001 (reprinted 2006).