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 



