Fitch appears to have started his Civil War career as Capt Cmd Co C of the Harrison County Home Guard from Sept 3 1861 to Sep 23 1861.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s881/1608.tif&Fln=S116319.pdf

He then goes to Capt. Cmdg, Co B Harrison County 6 month Volunteers from Oct 1861 to March 1862.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s901/0712.tif&Fln=S116320.pdf

He then ends up as a Corporal in the 57th EMM for a very short period entering August 21, 1862; resigning on Sept 12 1862.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s797/1190.tif&Fln=S116321.pdf

And finally enters the F&S group as a Capt Co A, 35th MO Infantry joining the unit a Benton Barracks St. Louis Sept 28, 1862. He is promoted to Lt. Col April 30, 1863 and is discharged June 28, 1865 Little Rock Arkansas.

http://www.sos.mo.gov/TIF2PDFConsumer/DispPDF.aspx?fTiff=/archives/AdjutantGeneral/Civil_War/ServiceCards/s863/0205.tif&Fln=S116322.pdf

The 1863/64 MO AG report list a very quick succession of Lt. Colonels that preceded Fitch taking the position.

http://digital.library.umsystem.edu/cgi/t/text/pageviewer-idx?c=umlib;cc=umlib;sid=5ac9d201fab7bc04cb7f4af855bd3f7b;q1=fitch;rgn=full%20text;idno=umlk000033;view=image;seq=316

A quick search of the OP only lists him as Lt Col of the 35th MO in tables of organization and a really odd entry where he apparently led 60 volunteers in the action against the Steamer Cotton in Bayou LaTeche and Berwick Bay under Gen. Weitzel. The report begins on page 234 of Series I Volume 15 (part I) and the reference to Horace Fitch comes on page 236 and is indexed as him at the end of the volume.

See

http://ehistory.osu.edu/osu/sources/recordView.cfm?Content=021/0234

Fitch's own note on the action of the 35th is reporte to the MO AG in 1866

Here
http://books.google.com/books?id=SAFAAAAAYAAJ&pg=PA257&lpg=PA257&dq=35th+Missouri+Helena&source=bl&ots=hQPXA94g-r&sig=vomKn0HICuDyp7mp3IPd0Q5BplQ&hl=en&ei=Af31S7HHBYX8Ndr_jYQI&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCIQ6AEwBA#v=onepage&q=35th%20Missouri%20Helena&f=false

The 35th was engaged in defending Hindman Hill and Battery D at Helena and was attacked by Fagan's troops. They lost 3 killed including Lt Onion and several wounded.

Ed Bearss work The Battle of Helena, July 4, 1863
The Arkansas Historical Quarterly, Vol. 20, No. 3 (Autumn, 1961), pp. 256-297
(article consists of 42 pages)
Published by: Arkansas Historical Association makes a number of references to the movements of the 33rd and 35 MO Inf during the battle.

Another interesting in sight into the 35th activities under Fitch is here in teh Missouri Senate Journal of 1863:

http://books.google.com/books?id=7qwbxlrjsfcC&pg=RA1-PA308&lpg=RA1-PA308&dq=35th+Missouri+Helena&source=bl&ots=QLqLpGBu9O&sig=aetMFgNoGAHMDEcUEv2CrOmvsEs&hl=en&ei=gwL2S_TNGYb4NcDF4IMI&sa=X&oi=book_result&ct=result&resnum=4&ved=0CB0Q6AEwAzgK#v=onepage&q=35th%20Missouri%20Helena&f=false

This site has a nice biographical summary of Horace Fitch, seems he contracted malaria during the war likely in Arkansas if not Helena well known to be one of the more unhealthy posts of the western theater.

http://www.rootsweb.ancestry.com/~moharris/hcbios.html#F

Hope it helps...

John R.