What category in my C.V. should I put papers where I've contributed but was not listed as an author, nor in acknowledgments? I performed a small set of experiments for a few published papers. The authors of these papers decided that my work was not significant enough to warrant my name on the papers. I am not mentioned in the acknowledgement section either, so my name isn't anywhere on the papers. (I left the group due to a lack of funding before they published the papers.)

I'm currently updating my CV and I'm wondering what category I should include these papers under. It should probably be separate from the "Publications" section where I am authors of those papers.

The field is in Biology and Biochemistry. Everything is done in the U.S., in a nationally-recognized lab.

Update: Thanks for all the answers and discussion. I am planning to just describe the experiments I did under "Work Experience" and add an author of the papers as a reference, in case there is a need for verification.
I don't think there's any good way to include these papers on your CV.  

It's not conventional to list anyone else's papers on your CV, so by default everyone will assume you are one of the authors of each paper you list.  If you aren't exceptionally clear, so clear that nobody could possibly misunderstand even if they are just skimming and not reading carefully, then you could easily upset people and give the impression that you are misleadingly inflating your publication record.

I think you could probably write it clearly enough (e.g., a section called "Papers in which I am acknowledged but not an author" or something equally explicit, in a substantially different location in the CV from your own papers).  However, drawing too much attention to this unconventional choice would itself be a problem.  You want the most attention-getting part of your CV to be your own accomplishments.  If readers get distracted by something strange, then it will work against the primary purpose of the CV.

These papers would count for almost nothing on your CV anyway, so I see no purpose to including them.  In fact, including something like this could look bad, by suggested a desperation to list as much as possible, regardless of how meaningful or important it was.  The one case I can think of in which it might be a good idea is a student applying to grad school, with no publications yet but with acknowledgments in a paper or two.  Then highlighting these contributions would at least be better than saying nothing.  (But it's not worth it if you are an author on other papers.)

As I was finishing this answer, I noticed the comment that no mention is made of this contribution in the acknowledgments.  In this case, the CV is absolutely the wrong place to deal with this issue, since it doesn't give an opportunity to clarify.  Intentionally omitting someone who carried out experiments from the acknowledgments is unethical, and doing is accidentally is a serious oversight, but your CV is not a good place to accuse someone of poor ethics or sloppiness.  (Without an explanation, nobody would have any idea what to make of this section of your CV.)