Populations of sea urchins often fluctuate greatly in density. The density of sea urchins is usually inversely correlated with the abundance of macrolagae. It is argued that fluctuations in density are a predictable consequence of the unique morphology and physiology of sea urchins. An extraordinary ability to accomodate food Limitation through variations in growth rate means that surv~vorship ...

Two new echinoid genera and species, Salvaster roberti gen. et sp. nov. and Pygolampas edita gen. et sp. nov. are described. They were collected in the Calcaires à Spatangues Formation (CSF) that consists of limestone and clay sediments deposited in the southeast of the Paris Basin (France) during the Early Hauterivian (Early Cretaceous). The CSF is dated from the Acanthodiscus radiatus chronoz...

X-ray diffraction studies confirm that, with few exceptions, each skeletal element of echtinoderms is a single crystal of magnesium-rich calcite and that a relation exists between the shape of the element and the crystallographic a- and c-axes. The exceptions incluide the teeth of echinoids, and the calcareous ring as well as the anal teeth of holothurians. The tubercles of an echinoid plate be...

Based on new material from Germany and Spain, the echinoid “Lepidocentrus” ibericus from the Early Devonian (Emsian) of northern Spain is shown to be congeneric with Rhenechinus from the Hunsrück Slate of south−western Germany. New in− formation on the lantern, pedicellariae and internal structure of the theca is provided, and confirms this genus as a member of the Echinocystitidae–Proterocidar...