Flowering stone plant

This is one of my older stone plants (Lithops). Three flowers opened just today. I have lost the label for this plant, so I can't remember what species it is, but I should be able to work it out from my list of plants that I keep for a record of how they grow, when they divide etc.

The flowers start out being rather spindly, and each evening they close up. The next day when they open again, the petals have grown a little, and over time the flowers become gradually larger.