_Porcellio laevis_ 35 52. Flagellum and last peduncular joint of the antenna of _Porcellio ratzeburgii_ 36 53. Flagellum and last peduncular joint of the antenna of _Metoponorthus pruinosus_ 37 54. Flagellum and last peduncular joint of the antenna of _Metoponorthus cingendus_ 38 55. Flagellum and last peduncular joint of the antenna of _Cylisticus convexus_ 39 56. Flagellum and last peduncular joint of the antenna of _Armadillidium nasatum_ 40 57. Flagellum and last peduncular joint of the antenna of _Armadillidium vulgare_ 41 58. Flagellum and last peduncular joint of the antenna of _Armadillidium pulchellum_ 42 59. Flagellum and last peduncular joint of the antenna of _Armadillidium depressum_ 43 THE BRITISH WOODLICE. =Introduction.=--Having finished a somewhat exhaustive list of the land and fresh-water molluscs of Essex,[1] one of the present writers felt that if he were to make any further contributions of importance to a knowledge of the fauna of that interesting county, he must turn his attention to some other group of animals. It seemed most fitting that some creatures should be chosen which are commonly met with during the search for molluscs. Centipedes, millepedes, and woodlice fulfilled these conditions, and all were collected, but as only seventeen species of woodlice had at the time been found in England, it was deemed advisable to study these in detail to begin with. The present contribution is the result of the undertaking, and we have thought that a general consideration of the British Woodlice, with careful drawings from nature of all the species now known from this country, ought to lead to a more general study of these interesting creatures and their habits. =Position in the scheme of classification.=--The Woodlice belong to an immense group of invertebrate animals known as the Arthropoda, the bodies of which are segmented and provided with jointed appendages for purposes of walking, swimming, and feeding. Of this group, two large divisions are recognized. The first contains the forms which breathe by means of air-tubes, such as the Insects; and the second has been constituted for Crustacea, which breathe by means of gills. The latter are, of course, adapted more especially for a life in water, but here and there we come across examples so modified that they can exist in air. The land-crabs are a case in point, and so are the Woodlice. These belong to an order which contains many fresh-water and marine species, known as the Isopoda.