Two names haven't been mentioned yet:
Alfred North Whitehead (1861 - 1947)
Bertrand Russell (1872 - 1970)
While Whitehead and Russell were not principally mathematicians, each published works in the field prior to their collaboration on Principia Mathematica (1910 - 1913; 1927); e.g.,
Russell, An Essay on the Foundations of Geometry (1897) and Principles of Mathematics (1903), and Whitehead, A Treatise on Universal Algebra with Applications (1898).
