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 CRUSTACEANS The larger specimen will occasionally get on a piece of floating pumice, and sit there for a time. They are able to travel at a good rate on land, backwards and forwards. In the water they more often go backwards than forwards, probably that they may keep their enemies in sight.' ' It should be noted that, although the terms crayfish and crawfish are sometimes used indiscriminately and are in origin identical, it is now generally thought convenient to limit the use of the term crayfish to fluviatile genera and species, reserving the name crawfish for the ' rock- lobsters ' or Palinuridce, which are distinguished by their long and strong second antennse, and by the comparatively feeble development of their first legs. In a private letter, dated November 7th, 1900, Mr. Thomp- son obligingly supplements the above extract as follows : ' The Crayfish occurs in many places. I have known it to occur near St. Andrew's Mill, close to the town of Northampton, for forty years. Other places quoted by Mr. H. F. Tomalin {yourn. North. Nat. Hist. Soc, vol. iv. p. 242) are Bazeley's Mill (below Upton Mill) ; Cosford's Mill, Heyford ; Milton Brook ; Tecton Brook, below Holdenby Mill ; the Tove below Castlethorpe ; and at Castlethorpe and Stoke Bruerne.' When Huxley prefaces his wonderfully elaborate study by referring to the subject of his treatise as ' one of the commonest and most insignificant of animals,' his words must not be taken too literally. For in Northamptonshire, as in other counties, there are assuredly many other malacostracan Crustacea both commoner and much more insignificant than crayfishes. Thus of the isopod, Asellus aquaticus (Linn.), Bate and Westwood write, ' This is a very common animal, occurring in fresh- water ponds and ditches throughout the kingdom.'^ It is half an inch long or less. About the same size is its constant companion, the amphipod, Gammarus pulex. No less certainly than these two, may the terrestrial isopods, Oniscus asellus and Porcellio scaber, be relied on as belonging to the fauna of the county. These, with others of their kith and kin, vulgarly known as woodlice, slaters, sows, or pigs, though they are true crustaceans, have only not been recorded because so com- monly things common are accounted to be things of no account. The freshwater entomostraca have not been so entirely neglected, though these in the number of species on record must be very far below the number of species in existence. The saying that ' Northampton- shire gives water to all surrounding counties, and receives none in return'^ might suggest a reason for comparative poverty in water-fleas. But this is not a reason on which it would be safe to rely without direct investiga- tion, since the distribution of entomostracan Crustacea does not exclusively depend on direct transmission by water. They are often to be found in situations to which no existing streams could have carried them. Among the species definitely recorded are one or two well deserving attention. The first in systematic position belongs to a set of creatures ' Journal of the Northampton Natural History and Field Club, vol. iv. p. 1 72 (December, 1 886). ' Journ. Northampton Nat. Hist. Soc, vol. i. p. 48 (1880-81). 103
 * British Sessile-eyed Crustacea, vol. ii. p. 314.