Searching for Thomazine among Geni Profiles, I located a Thomazine Sheffield born 7/17/1610, in Sudbury, Suffolk. No history is known. While Thomazine seems an uncommen name today, it was not so much so in 17th Century England. Also, Sudbury was a long way from Somerset in those days.
While Thomazine's last name could be Sheffield, there is not suffiecient evidence to support a conclusion.