These buttons are in the shape known as a "trefoil" design. By definition, the word trefoil means: a lobed, cloverlike leaf on a small European plant of the pea family. I don't own many trefoil buttons, but I'm always on the lookout for these rarities. It's interesting to note that back in the 1920's, when Bakelite first came on the market, the Bakelite company used a trefoil logo in magazine ads.
In these vintage ads the company catchphrase is, "BAKELITE: The Material of a Thousand Uses."