Trilobites seem to have been exclusively marine, since they are found only in association with the remains of marine animals. Their range in depth was evidently considerable, for they occur in many different kinds of sediment, and were apparently able to live regardless of the nature of the sea-floor—whether muddy, sandy, calcareous, or rocky. In some cases they occur in deposits containing reef-building corals and other shallow water animals; in others they are associated with organisms which lived at greater depths. The group appears to have had a world-wide distribution, for the remains of Trilobites are found in the Palaeozoic rocks of all countries. Their range in size is considerable; for whilst a large proportion of the species are about two or three inches in length, some, like _Agnostus_, are only a quarter of an inch long, others are from ten to twenty inches long, the largest forms including species of _Paradoxides_, _Asaphus_, _Megalaspis_, _Lichas_, and _Homalonotus_. The feature in a Trilobite which first attracts attention is the marked division of the dorso-ventrally flattened body into a median or axial part, and a lateral or pleural part on each side. It was this character that led Walch, in 1771, to give the name by which the group is now known. The axial part of the body contained the alimentary canal, as is shown by the position of the mouth and anus, as well as by casts in mud of the canal which are found in some specimens. The trilobation of the body is quite distinct in the majority of Trilobites, but in a few genera belonging to the Asaphidae and Calymenidae (Fig. 136) it becomes more or less completely obsolete. [Illustration: FIG. 136.—_Homalonotus delphinocephalus_, Green, × 1. Silurian. (After Zittel.) ] In most cases the only part of the Trilobite which is preserved is the exoskeleton which covered the dorsal surface of the body. That skeleton consists largely of calcareous material, and shows in sections a finely perforated structure. Generally it is arched above, but in some cases is only slightly convex; in outline it is more or less oval. Three regions can always be distinguished in the body of a Trilobite—the head, the thorax, and the abdomen or pygidium.