The Arachnida are only very rarely found in a fossil condition. As far as is yet known, both the Scorpions and the true Spiders appear to have their commencement in the Carboniferous epoch, the former being represented by the celebrated Cyclophthalmus senior from the coal-measures of Bohemia, and by the Eoscorpius carbonarius of the Carboniferous strata of Illinois. Other Carboniferous Arachnida have been referred to the genera Eophrynus, Architarbus, and Mazonia. Spiders are also known to occur in the Jurassic rocks (Solenhofen Slates) and in the Tertiary period. The Mites, Harvest-spiders, and Book-scorpions have been detected in amber.