The full report is available online at Brian Downey's encylopedic Antietam on the Web. 
In it, Howard describes the battle in the West Woods three days earlier.
At the very end, he gives the tally of the division's casualties that day. Of those wounded, 15% to 20% would die within days, months, or even years of their injuries.
"The total loss of the division is as follows: 
|General Gorman's Brigade||134||536||88|
|General Dana's Brigade||128||650||124|
|General Burn's (or Howard's) Brigade||89||370||109|
|Company A, 1st Rhode Island Artillery||4||15||----|
|Company I, 1st United States Artillery||----||6||----|
What about the 321 men from that division who went missing that day? In an earlier post, five missing from the 72nd Pennsylvania were identified. Who were the others? What became of them?
In the next year, I would like to assemble a list of the missing of the West Woods, both Union and Confederate. This will not be an easy task but I figure it is worth a try. Hopefully, readers may have information as to names and units, and, if possible, information on what became of these casualties of the West Woods.
Antietam National Cemetery
Number 3826, an unknown soldier from Pennsylvania  one of over 1,700 unknowns buried in the National Cemetery.
He lies next to Pvt. Henry W. Dean, of Company B, 12 Pennsylvania Reserves (Harrisburg) who was probably killed in the cornfield early on September 17. 
 BGen. Oliver O. Howard's Official Report (OR), September 20, 1862 retrieved from Brian Downey's website Antietam on the Web at http://aotw.org/exhibit.php?exhibit_id=53.
 History of Antietam National Cemetery including a descriptive list of all the loyal soldiers buried therein together with the ceremonies and address on the occasion of the dedication of the grounds, September, 17th, 1867 (Published 1869) retrieved from Western Maryland Historical Library online at http://www.whilbr.org/itemdetail.aspx?idEntry=1211&dtPointer=0.
 Map of Antietam National Cemetery as designed by A. A. Biggs, M. D., President & Genl. Supt., Sharpsburg, 1866. Retrieved from Western Maryland Historical Library online at http://www.whilbr.org/itemdetail.aspx?idEntry=1266&dtPointer=0.