Members of the family Siphonorhinidae Cook, 1895 are thread-like eyeless millipedes that possess an astounding number of legs, including one individual with 750. Due to their cryptic lifestyle, rarity in natural history collections, and sporadic study over the last century, the family has an unclear phylogenetic placement, and intrafamilial relationships remain unknown. Here we report the discovery of a second species of Illacme, a millipede genus notable for possessing the greatest number of legs of any known animal on the planet. Illacme tobini sp. n. is described from a single male collected in a cave in Sequoia National Park, California, USA. After 90 years since the description of Illacme, the species represents a second of the genus in California. Siphonorhinidae now includes Illacme Cook & Loomis, 1928 (two species, USA), Kleruchus Attems, 1938 (one species, Vietnam), Nematozonium Verhoeff, 1939 (one species, South Africa) and Siphonorhinus Pocock, 1894 (eight species, India, Indonesia, Madagascar, Vietnam). [ABSTRACT FROM AUTHOR]