Unfortunately you have not given enough information to be able to identify Mary Perry, a common name. There is no Gellin or Farish in Wright's book of William Browning Greene and his wife Mary Hoxie Lewis. Need to know everying about this Mary Perry that you know. What does the 1850 census say about her? Be aware that it was not uncommon for folks with the same name to infer kinship without any valid connection. Or if there were, it was tenuous at best. What about her children's names? They can provide clues. Look for "Hazard", "Benjamin", "Samuel", "Simeon","Edward" and other family names common to Perry's of Rhode Island.