The synthesis of many years' research carried out by members of the British Phycological Society in collaboration with The Natural History Museum, this is the eighth in a series of books being published under this title covering all the British and the majority of the northern Atlantic seaweeds. This volume is stylistically similar to earlier titles in the series and is written by leading authorities in red algae. The sub-class and all orders, families, genera and species are described, notes on ecology and distribution given, and each named species is illustrated by one or more half-tones showing distinctive features. User-friendly keys are provided to enable identification of genera and species. Special features of this volume are the comprehensive introduction and complete re-evaluation of the British Isles taxa of the sub-class, including a revision of the species of Porphyra.