In 1820, Israel Clark and Hannah Willis name a son, James Harvey Clark, and he appears with them on the 1850 Census, for Johnson Co., Iowa.
They migrated from Preble Co OH>Fountain Co., IN >? ILL >Iowa> and on to Oregon, where James and his mother are killed in a sad robbery and house burning.I do not see where he had been married or had children.
It is possible that your James H. Clark, could be a younger brother to Israel L. Clark and my Jonathan E. Clark and Deborah Clark Bridge.
He could be James Clark, who marries Aug. 01, 1819, Preble Co OH, to Keziah Davis.Charles Hart points out that the same minister, James Crawford, also performed the marriage of Israel L. Clark and Hannah Willis, in 1815.
There is a Ohio land record, where Benjamin Willis, father? of the wife of Israel, transacts to Abner Willis, and it is Witnessed by Israel L Clark and JAMES H. CLARK.
SEE MESSAGE 963 in this Forum, by Charles Hart
9-9-1829 - Benjamin WILLIS Sr. of Preble to Abner WILLIS or Preble; $150; Washington Twp; 100 acres. Line - Samuel SHOWALTER; S12 T7 R2 SE1/4 , State Line. Wit. Israel L. and James H. CLARK. recorded 10-24-1829 page (156)
I doubt this James H. Clark is Israel's 9 year old son, but rather, a possible younger brother.I will take another look at the Early Census for Preble, and see again, what ages Stephanus Clark +45, showed for persons in household. ( I believe it is in this same thread here)