Conspicuous stipules are also visible. Triumfetta semitriloba, Sacramento bur. Note the five-merous perianth whorls. In this case the calyx is somewhat petaloid, at least on the front surface. In this species the perianth is 4-merous and the sepals are petaloid.
Numerous stamens are evident.
This photo clearly shows the valvate nature of the calyx that is typical for this family. The petals are somewhat smaller than the sepals. Numerous brownish stamens are nestled around the ovary. Note numerous stamens, sepals petaloid on front, green on back.