While visiting tropical forests in many parts of the world I have been struck by the abundance of crustose lichens belonging to the genus Opegrapha, but unable to progress far in their identification. This study by Damien Ertz has arrived as a welcome addition to our knowledge of the genus, where 45 species, eight of which are new, are figured and described from the palaeotropics. Ertz has taken a traditional approach to the taxonomy of the species’ following, but also building upon the work of Redinger and others. He provides a short but informative introduction to the genus, providing a brief history of previous monographs and collectors. There follows a section on morphology and chemistry of the taxa and a brief note on their ecology. A set of well written dichotomous keys follows, the primary division being made on spore septation. I was pleased to see that ascospore size ranges were based on the standard deviation, a sensible approach that should be much more widely adopted by lichenologists. The species descriptions are full, appear to be accurate, and include good monochrome close-up photographs of the apothecia, diagrams of ascospores and asci, and distribution maps for many taxa.

I tested the key with two of my own African collections and managed to identify O. bonplandii and O. viridis without any difficulty, although the latter did not much resemble typical O. viridis from the UK. Ertz recognised that the latter is a variable taxon, rather like O. varia in Europe, and Ertz agrees that some further critical studies will be needed. A few of his taxa, such as O. vulgata are referred to as O. vulgata sensu lato. It appears that none of the tropical material he examined possessed conidia. This poses a problem since several Opegrapha species are identified on the shape and size of their conidia, including O. vulgata. Using a broader species concept appears reasonable in this situation however. Ertz has synonomized a good number of species originating from the palaeotropics and found that several type specimens were so degraded or scanty that they could not sensibly be identified. This is a common problem and emphasises the importance of careful, extensive sampling in the tropics. Over 20 Opegrapha species occur on trees in the UK alone, so it is likely that the number of taxa in the tropics will be far higher than the 45 species listed by Ertz, but some well known British species such as O. varia and O. herbarum seem to be widespread in the tropics and are perhaps cosmopolitan.

Overall I found this work to be well researched and written and I only have few quibbles. The first concerns O. prosodea, a species occurring in the southern parts of Britain and originally described by Acharius. Ertz mentions this species in connection with the similar O. viridis but does not mention it as occurring in the palaeotropics. However, in the description of the lichen by Acharius, its provenance was given as Sierra Leone, and some mention should have been made of this. The second relates to O. astraea. In the keys, this taxon appears twice under two different sections. Usually, if a taxon appears more than once in a key, it is because it is not a good species or the key is deficient. He notes in the description that the species is very variable and I suspect that we may discover further synonyms of species in the future as our understanding of this interesting genus grows.

I found this work well laid out, easy to read and use and well referenced. I recommend it without hesitation to all lichenologists with an interest in tropical crustose lichens.