Bollmaniulus sp. (Parajulidae, millipede), Mount Diablo bioblitz, Contra Costa County, California - 12 April 2014. Thanks to Rowland Shelley and Bug Guide for the identification: bugguide.net/node/view/908534#1619795
On Pioneer Tree trail
Was in very moist ground under a rock
Millipedes are myriapods of the class Diplopoda that have two pairs of legs on most body segments. Each double-legged segment is a result of two single segments fused together as one (the name "Diplopoda" comes from the Greek words διπλοῦς (diplous), "double" and ποδός (podos), "foot"). Most millipedes have very elongated cylindrical or flattened bodies with more than 20 segments, while pill millipedes are shorter and can roll into a ball, like a pillbug.