Numerous texts from early 1900's, list Henry's wife, as Mary Newland, sister of William, so I imagine this is repeated in numerous pedigrees. There are just to many texts that have this to even list/cite them here.
She is referred to as Mary (unk) in the Great Migration and recent Mayflower Descendant articles.
Anderson, Robert C. Great Migration Begins: Immigrants to N.E. 1620-1633, Vols. I-III p. 1018 listed as Mary Unknown
Mayflower Descendant V 54 p 97 2005 lists Abigail Howland, daughter of Henry and Mary (Unk) Howland.
I don't have access to: National Genealogical Society Quarterly 1987: V. 75: 107 Which seems to be a recent and frequently cited reference to Henry Howland.
Seems she should probably be Mary (Unknown), with possibly Newland in a dispute section. .