In the tropics lichens come under the influence of many climates: on the high mountains there is a region of perpetual snow, lower down a gradual change to temperate and finally to tropical conditions of extreme heat, and, in some instances, extreme moisture. There is thus a bewildering variety of forms. By “tropical” however the warmer climate is always implied. Several families and genera seem to flourish best in these warm moist conditions and our familiar species grow there to a large size. Among crustaceous families Thelotremaceae and Graphidaceae are especially abundant, and probably originated there. In the old comprehensive genus _Graphis_, 300 species were recorded from the tropics. It should be borne in mind that _Trentepohlia_, the alga that forms the gonidia of these lichens, is very abundant in the tropics. _Coenogonium_, a genus containing about twelve species and also associated with _Trentepohlia_, is scarcely found in Europe, except one sterile species, _C. ebeneum_. Other species of the genus have been recorded as far north as Algeria in the Eastern Hemisphere and Louisiana in the Western, while one species, _C. implexum_, occurs in the southern temperate zone in Australia and New Zealand. Of exclusively tropical lichens, the Hymenolichens are the most noteworthy. They include three genera, _Cora_, _Corella_ and _Dictyonema_, the few species of which grow on trees or on the ground both in eastern and western tropical countries. Other tropical or subtropical forms are _Oropogon loxensis_, similar to _Alectoria_ in form and habit, but with one brown muriform spore in the ascus; it is only found in tropical or subtropical lands. _Physcidia Wrightii_ (Parmeliaceae) is exclusively a Cuban lichen. Several small genera of Pyrenopsidaceae such as _Jenmania_ (British Guiana), _Paulia_ (Polynesia) and _Phloeopeccania_ (South Arabia) seem to be confined to very hot localities. On the other hand Collemaceae are rare: Wainio records from Brazil only four species of _Collema_, with nine of _Leptogium_.