William Diller Matthew FRS (February 19, 1871 – September 24, 1930) was a vertebrate paleontologist who worked primarily on mammal fossils, although he also published a few early papers on mineralogy, petrological geology, one on botany, one on trilobites, and he described Tetraceratops insignis, which was much later suggested to be the oldest known (Early Permian) therapsid.Matthew was born in Saint John, New Brunswick, the son of George Frederic Matthew and Katherine (Diller) Matthew.