CRUSTACEANS reaches to the far end of the fifth or antepenultimate joint, whereas in N. aquilex it reaches not nearly so far. In like manner, if the terminal segment of the body, the segment which has no appendages and is known as the telson, be detached and flattened out, it will be seen that the median slit runs much further up in N. kocbianus than in N. aquilex. It is perhaps worth suggesting that dissections should not be practised upon rare specimens until some skill has been acquired by dealing with examples that are common and plentiful, such as those of Gammarus pulex. It is rather singular that Dr. Hamilton should have mentioned the universally prevalent freshwater amphipod, without making any allusion to its almost equally common and very frequent companion, our fresh- water isopod, Asellus aquaticus (Linn.). The genuine Isopoda are sessile-eyed like the Amphipoda, with which they further agree in having the seven segments of the thorax or middle body articulated and not covered by a carapace. But they differ from the amphipods and from almost all the other Malacostraca in one highly important particular. They have the appendages of the abdomen or pleon modified for branchial purposes, in this respect agreeing only with the small group of the Stomatopoda or Squillidas. But whereas the latter have the abdomen enormously developed, this portion in the Isopoda is comparatively reduced, often with the seven segments all consolidated, and uniformly with the sixth and seventh segments united into one piece so that there is no separate telson. The presence of A. aquaticus in the streams of Berkshire may be taken for granted. The presence of all our commonest English woodlice in its roads and gardens, woods and hedge- rows, may with equal confidence be presumed. There cannot be any reasonable doubt that the county harbours Philoscia muscorum (Scopoli) ; Trichoniscus pusillus, Brandt ; Oniscus asellus, Linn. ; Platyarthrus hoff- mannseggii, Brandt ; Porcellio sca&er, Latreille ; and Armadillidium -vulgare (Latreille). These form rather more than a fourth part of the whole number of species which England can at present claim among the terrestrial isopods, but even of these few the existing records only point the finger at Berkshire without definitely naming it. The six species mentioned, being generally distributed in neighbouring and surrounding counties, could have no motive for omitting this one from the pertinacity of their colonizing instincts. The little opaque-white Platyarthrus^ which lives in ants' nests, has been observed not many miles from the county boundary, and a seventh species, Metoponortbus pruinosus (Brandt), is re- corded as ' plentiful in the vicinity of Oxford,' 1 an expression capable of including both the shires whose borders are illumined by that learned city. Of Oniscus asellus and Porcellio scaber Miss Slocock has kindly sent me specimens collected in the wood at her father's residence, Goldwell, Newbury. Like the Malacostraca at large, the air-breathing isopods have two pairs of antennas, but the first pair are small and obscure in those terrestrial crustaceans commonly called woodlice, whereas the second 1 Brit, Sets. Crust, pt. 21 (1868), ii. 488. I 129 17
