Anna was spot on when she guessed last week's mystery seeds were from blackberries, Rubus sp.
Unfortunately my photographs did not show the details found in close ups of mature seeds.
Check out the ridges and grooves in the surface of these black raspberry seeds, Rubus occidentalis (Photograph by Steve Hurst @ USDA-NRCS PLANTS Database). Don't they look a bit like sponges?
Blackberries grow on shrubby plants. The stems are covered with prickles.
Various related species grow throughout Europe and North America.
The flower is white with five petals.
When the flower is pollinated, the petals drop off and the drupelets begin to ripen.
All this comes from a single flower.
Are blackberries ripe yet where you live?
Do you have a favorite recipe for blackberries?