Cryptolithus, genus of trilobites (extinct arthropods) found as fossils in Europe and North America in the Ordovician period (505 million to 438 million years ago). Its distinctive appearance makes the genus a useful guide fossil for Ordovician rocks and time. The head region, or cephalon, in Cryptolithus is large, with margins pitted in a distinctive pattern; two long spines project back from the lateral margins. The adults lack eyes.