Sedimentary structures, for example those produced by empty shells rolling along the sea floor, are not produced through the behavior of an organism and not considered trace fossils. The study of traces is called ichnology, which is divided into paleoichnology, or the study of trace fossils, and neoichnology, the study of modern traces. This science is challenging, as most traces reflect the behavior--not the biological affinity--of their makers. As such, trace fossils are categorized into form genera, based upon their appearance and the implied behaviour of their makers.