Illustration by James Sowerby for James Edward Smith's English Botany. There are more details about the production of English Botany in the INTRODUCTION
These figures are taken from plate 2298 and the species shown here is known today as Sticta sylvatica. In the comments that accompanied the plate Smith wrote that the underside was "...besprinkled with whitish pits, among short soft down...". Those pits are technically called cyphellae and are a distinctive feature of the genus Sticta.