Dubois's complete collection of fossils were transferred between 1895 and 1900 to what is now known as the Naturalis, in Leiden in the Netherlands. The main fossil of Java Man, the skullcap cataloged as "Trinil 2", has been dated biostratigraphically, that is, by correlating it with a group of fossilized animals (a "faunal assemblage") found nearby on the same geological horizon, which is itself compared with assemblages from other layers and classified chronologically. Ralph von Koenigswald first assigned Java Man to the Trinil Fauna, a faunal assemblage that he composed from several Javanese sites. He concluded that the skullcap was about 700,000 years old, thus dating from the beginning of the Middle Pleistocene.