I know that William H. Clements was part of the Mississippi Rifles from a military pension record index entry. Specifically, he served 1846-1847 in Company C.
According to the Army National Guard Special Designation Listing, "Mississippi Rifles" was a special designation for 155th Infantry Regiment, also known as "First Mississippi".
I found his military grave registration. I would like help transcribing the bottom of the page:
I can read some of it:
Warren Co 11. Date Enlisted 1846 Date Discharged 1847 12. Service Army (Mex) Rank Private 13. Company, outfit or ship Co C. 1st Miss Inf. " ------------ Southerners"
What is the missing word?