Darrell,

The only J. S. Shattuck I could find in a Texas unit was this man.

John S. Shattuck (First_Last)
Regiment Name Baird's Reg't Texas Cavalry
Side Confederate
Company FS
Soldier's Rank_In 1 Lt & Adjt
Soldier's Rank_Out 1 Lt & Adjt
Alternate Name
Notes
Film Number M227 roll 33

If your man was James Shattuck then this may be your man.

J. S. Shattuck (First_Last)
Regiment Name Powers' Regiment Confederate Cavalry.
Side Confederate
Company F&S
Soldier's Rank_In Lieutenant
Soldier's Rank_Out Capt. & A.Q.M.
Alternate Name James W./Shattuck
Notes
Film Number M818 roll 21

James W. Shattuck (First_Last)
Regiment Name General and Staff Officers, Corps, Division and Brigade Staffs, Non-com. Staffs and Bands, Enlisted Men, Staff Departments, C. S. A.
Side Confederate
Company
Soldier's Rank_In A.Q.M.
Soldier's Rank_Out
Alternate Name
Notes
Film Number M818 roll 21

I hope this helps,
Gary D. Bray