Interesting cartridge. Here is a link for some rimfire cartridges. I am guessing one of the Spenser cartridges. BUT probably wrong!!
http://en.wikipedia.org/wiki/List_of_rimfire_cartridges
Turtlefoot beat me to it. It appears to be the earliest version which was a bar primed. If there is an indentation about an 8th of the way up from the base it was Benet primed and made a tad bit later.
Those are great finds. Most are found smashed and that was a directive from the Secretary of War because the indians were finding a way to reload them. Soldiers were commanded to stomp on them.