Sorry... I'm going to reply and not wait for PBK... he'll come along and correct me on something I'm sure!
Enlisted Collar Insignia
Complaints in 1907 about the loss or snagging of officer cut-out collar insignia led to the adoption of bronze circular disks for enlisted personnel. Die struck with the branch, US and unit designation, the collar ornaments changed to gilt after 1924.
Cryptodave is correct! U.S. Great Seal Button (Your's looks like collar brass as well)
With the adoption of the Great Seal in 1783, Army buttons begin to reflect it. By 1854 all enlisted personnel wore -- and after 1902 all personnel had -- the Great Seal button, except engineer officers, who retained their distinctive branch button.
Eagle and Anchor button... US Marines... looks like some late 1800s versions, but I cannot verify the year with out better pics from you... see this link:
http://www.thetreasuredepot.com/cayenna/index.html
US Army Saint Christopher medalian... the good Saint Christopher carrying the boy accross the river.... thats what it looks like anyway - with the Army name added. Check this out:
http://www.jewelbasket.com/jbuur41580.html and
[link removed at request of site owner]
Unmarked brass star/badge?