. Bulletin of the British Museum (Natural History), Geology. ARENIG OF CARMARTHEN DISTRICT 239 communities, but that they are characteristic of the graptolitic latest Arenig and Llanvirn sedi- ments succeeding the Carmarthen Formation. We disagree with Bergstrom's (1973 : 45-46) contention that olenids were pelagic. A pelagic habit is unlikely because of the strict facies dependence of the Olenidae, indicating that their life habits were closely tied to bottom or near-bottom conditions. Also they exhibit a mutually exclusive relationship with the benthic trilobites of the Raphiophorid and Nese
. Bulletin of the British Museum (Natural History), Geology. ARENIG OF CARMARTHEN DISTRICT 239 communities, but that they are characteristic of the graptolitic latest Arenig and Llanvirn sedi- ments succeeding the Carmarthen Formation. We disagree with Bergstrom's (1973 : 45-46) contention that olenids were pelagic. A pelagic habit is unlikely because of the strict facies dependence of the Olenidae, indicating that their life habits were closely tied to bottom or near-bottom conditions. Also they exhibit a mutually exclusive relationship with the benthic trilobites of the Raphiophorid and Neseuretus Communities, as well as the Nileid and Illaenid-Cheirurid Communities in Spitsbergen (Fortey 1975a : 336). A pelagic trilobite is independent of benthic assemblage types (vide Carolinites Fortey 1975a : 343; Opipeuter Fortey 1974a: 123; Microparia this paper, p. 256). Furthermore, the morphology of the large flattened olenid Hypermecaspis venerabilis sp. nov. is surely typical of a benthic animal. Both this morphological type and the more convex, Bienvillia-type of morphology may be inter- preted as adaptations to cope with life on muddy sediments deoxygenated at, or just below, the sediment surface (Fortey 1974b: 11). Finally, Childress (1971) has shown that the Recent mysid Gnathophausia ingens can cope with minimum oxygen conditions because of an increase in the surface area of its oxygen-absorbing surfaces. Many Olenidae, such as Hypermecaspis herein, and a number of other forms of other families convergent with Olenidae (Fortey & Owens 1975 : 237), have wide (trans.) thoracic pleurae and a large number of thoracic segments, which is consistent with both lateral extension and multiplication of gill lamellae, and hence more efficient oxygen absorption. In short both morphology and occurrence of Ordovician olenids suggest benthic habits, and much of the same evidence can be applied to the Cambrian representatives of the family. However, there are a few Cambrian ole
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