A HISTORY OF BERKSHIRE 



front of the carapace. The carapace is the great cephalothoracic shield 

 which covers the bases of the eye-stalks, antennae, mouth-organs and 

 legs. The orbits are excavated on either side of the rostrum. The 

 transversely sutured telson is at the opposite extremity to the rostrum, 

 being the last of the twenty-one segments which may be assumed as 

 constituents of the head, thorax and abdomen (or cephalon, perason and 

 pleon) of a stalk-eyed malacostracan. The gills or branchiae are limited 

 to the head and thorax, and are concealed under the carapace. They are 

 called podobranchias when attached to the basal joint of an appendage, 

 arthrobranchiae when attached to the membrane connecting this joint 

 with the supporting segment, and pleurobranchia? when attached to the 

 side of the segment itself. In Potamobius there are six pairs of the first 

 sort, eleven of the second, and of the third one pair well developed and 

 either two or three pairs that are rudimentary. The reckoning of 

 twenty-one segments is spoken of as an assumption, because out of the 

 first fourteen, which belong to the cephalothoracic division, all but the 

 last are here so firmly soldered together that their original individuality, 

 though not a matter of any reasonable doubt, is a matter of inference. 

 In regard to the distinctive characters above arrayed, a beginner will 

 very likely not find it especially easy to determine whether his specimen 

 has three pairs of rudimentary pleurobranchias or only two, but the other 

 differences are not difficult to observe. In the case of fresh specimens 

 assistance may be derived from the colouring of the limbs, these being 

 so much redder in one form than in the other that the noble crayfish 

 has been distinguished as the ' red-clawed ' from the ' white-clawed ' 

 stone crayfish. The value of this distinction is much enhanced by the 

 evidence adduced that, notwithstanding the provision of ample oppor- 

 tunity, the two forms do not interbreed. 1 That our English species is 

 the stone crayfish seems to be placed beyond doubt, but in distinguishing 

 that species as Astacus torrentium from the other and larger form which 

 he calls A. nobilis, Huxley has chanced to be unlucky in all his 

 names. From the more extended researches of Dr. Walter Faxon it 

 appears that the German name Steinkrebs, with its Latin equivalent 

 A. saxatilis, has covered two varieties or very nearly related species. 

 Out of much confusion the scientific names torrentium (Schrank, 1803) 

 and pallipes (Lereboullet, 1858) emerge as the earliest available names 

 for the forms in question, pallipes belonging to the one found in England. 

 On the other hand, the ancient name Jluviatilis, though under accepted 

 rules its date is restricted to its use by Fabricius in 1775, has still a long 

 precedence over nobilis (Schrank, iSoj). 2 For all three forms, as already 

 stated, the correct -generic name is Potamobius. 



As in other inland counties, the remaining crustaceans are all of 

 small size and of no commercial importance, though undoubtedly 

 valuable as consumers of waste products and as a food supply to animals 

 higher in general esteem. Conspicuous among the minor species is the 



1 Huxley, The Crayfish, ed. 3 (1881) p. 297. 



Faxon, Proceedings of the American Academy of Am and Sciences (1884), xx. 153-6. 



126 



