Thalline margin absent in apothecia 1. *Pyxine Nyl. Thalline margin present in apothecia. Thallus foliose 2. Physcia Schreb. Thallus fruticose 3. Anaptychia Koerb. C. *HYMENOLICHENS Fungus a Basidiomycete, akin to _Thelephora_. Algal cells _Scytonema_ or _Chroococcus_. Thallus crustaceous, squamulose or foliose. Spores colourless, produced on basidia, on the under surface of the free thallus. The Hymenolichens[1061] are few in number and are endemic in tropical or warm countries. They inhabit soil or trees. Thallus of extended lobes. Gonidia near the upper surface 1. *Dictyonema Zahlbr. Gonidia in centre of tissue 2. *Cora Fr. Thallus squamulose, irregular 3. *Corella Wain. II. NUMBER AND DISTRIBUTION OF LICHENS 1. ESTIMATES OF NUMBER Calculations have been made and published, once and again, as to the number of lichen species occurring over the globe or in definite areas. In 1898 Fünfstück stated that about 20,000 different species had been described, but as many of them had been proved to be synonyms, and since many must rank as forms or varieties, the number of well-authenticated species did not then, according to his estimate, exceed 4000. Many additional genera and species have, however, been discovered since then. In Engler and Prantl’s _Pflanzenfamilien_, over 50 families and nearly 300 genera find a place, but even in these larger groupings opinions differ as to the limits both of genera and families, and lichenologists would not all accept the arrangement given in that volume. Fünfstück has reckoned that of his estimated 4000, about 1500 are European and of these at least 1200 occur in Germany. Probably this is too low an estimate for that large country. Leighton in 1879 listed, in his _British Lichen Flora_, 1710 in all, and, as the compilation includes varieties, it cannot be considered as very far astray. On comparing it with Olivier’s[1062] recent statistics of lichens, we find that of the larger fruticose and foliose species, 310 are recognized by him for the whole of Europe, 206 of these occurring in the British Isles. Leighton’s estimate of similar species is about 145, without including varieties now reckoned as good species. In a more circumscribed area, Th. Fries[1063] described for Spitzbergen about 210 different lichens, a number that closely approximates to the 206 recent records by Darbishire[1064] for the same area. A general idea of the comparative numbers of the different types of lichens may be gathered from Hue’s compilation of exotic lichens[1065], examined or described by Nylander, and now in the Paris herbarium. There are 135 genera with 3686 species. Of these, about 829 belong to the larger foliose and fruticose lichens (including _Cladoniae_); the remaining 2857 belong to the smaller kinds, most of them crustaceous. 2. GEOGRAPHICAL DISTRIBUTION A. GENERAL SURVEY