Possibly the same person as Martha. Almost all the other children of Zachariah and Nancy were listed as only 8 years older in 1880 than they were listed as in 1870, so if the same thing happened to the Martha aged 6 months in 1870, one would expect her to be listed as age 8 or 9 in 1880, as Alice was. If they are not the same person, then Martha must have died by 1880.