Our mystery seeds stored by ants last week have a few common names, but let's use small prostrate sandmat. The scientific name is Chamaesyce prostrata (sometimes Euphorbia prostrata).
Here's another look at the wrinkly seeds (photograph by Carole Ritchie, hosted by the USDA-NRCS PLANTS Database.)
The sandmats or spurges have small, somewhat oddly-structured flowers that attract ants. (more about Euphorbia flower structures). At least some of the sandmats are likely pollinated by ants (see post at Wild About Ants).
Back to the mystery post, why would ants want to store the sandmat seeds?
It turns out the sandmat seeds have a structure that serves as food for ants called a caruncle. The ants bring the seeds back to the nest, pull off the caruncle and eat it, and then discard the rest. (A more generalized term for this kind of structure on a seed is an elaiosome. ) That is why sandmat often grows around ant nests.
Do sandmats or ground spurges grow where you live? What common name do you use?