Bullaces are small, 'wild' plums. They are much smaller than cultivated plums and more spherical than the oval of damsons, greengages or the related mirebelle plum, also often seen growing wild. When ripe the the black bullace is a deep wine red colour. This is also when they are sweet enough to eat raw.
They are a sub-specious of prunus domestica from which the many plum cultivars, such as 'Victoria', are derived. There are different forms of bullace which varying colours but the common wild type is the black bullace.
In some of the drove paths I walk there are neglected stretches of old mixed hedging and it is in such settings that you are more likely to come across bullaces. Ever vigilant for such opportunities I saw a tree with uniformly ripe bullaces within arms length. The small haul made a contribution to a fruit salad later in the evening.