Hyphens aren't allowed, v-nigrum apart.
Is the Key's reference to G. Gray 1856 correct? Murray Bruce and Normand David (Spelling, authorship, and date of the name of the Pacific Eider (Somateria mollissima v-nigrum), The Auk, Volume 124, Issue 2, 1 April 2007, Pages 709–712) suggest a different OD.