Among the many interesting groups of fossils found in the Palaeozoic deposits there is none which has attracted more attention than the Trilobites. As early as 1698, Edward Lhwyd, Curator of the Ashmolean Museum in Oxford, recorded in the _Philosophical Transactions_ the discovery of Trilobites in the neighbourhood of Llandeilo in South Wales; and of one of his specimens he remarked that “it must be the Sceleton of a flat Fish.” In the following year the same writer gave in his _Lithophylacii Britannici Ichnographia_ descriptions and figures of two Trilobites which are evidently examples of the species now known as _Ogygia buchi_ and _Trinucleus fimbriatus_. Although Trilobites differ so much from living Arthropods that it was difficult to determine even whether they belonged to the Crustacea or the Arachnida, yet one of the earliest writers, Dr. Cromwell Mortimer, Secretary of the Royal Society (1753), recognised their resemblance to _Apus_ (see pp. 19–36). This view of their affinities was adopted by Linnaeus, and has been supported by many later writers. Another early author, Emanuel Mendez da Costa, thought that the Trilobites were related to the Isopods, an opinion which has been held by some few zoologists of more recent times. The Trilobites form the only known Order of the Crustacea which has no living representatives. They are found in the oldest known fossiliferous deposits—the Lower Cambrian or _Olenellus_ beds, where they are represented by 19 genera belonging to the families Agnostidae, Paradoxidae, Olenidae, and Conocephalidae. From the variety of forms found and the state of development which they have reached, it is evident that even at that remote period the group must have been of considerable antiquity; but of its pre-Cambrian ancestors nothing is yet known; consequently there is no direct evidence of the origin of the group. Trilobites form an important part of all the faunas of the Cambrian system; they attain their greatest development in the Ordovician period, after which they become less numerous; their decline is very marked in the Devonian, in which nearly all the genera are but survivals from the Silurian period; in the Carboniferous, evidence of approaching extinction is seen in the small number of genera represented, all of which belong to one family—the Proëtidae, in the relatively few species in each genus and in the small size of the individuals of those species. In Europe no representatives of the group appear to have survived the Carboniferous period, but in America one form has been recorded from deposits of Permian age.