64 A HISTORY OF EECENT CRUSTACEA 



wrist of the chelipeds lias a long sharp crest aid the 

 rapid rubbing of this crest against the stride produces a 

 shrill sort of stridulating noise such as a grasshopper makes 

 by drawing the thighs of its hind legs over the salient 

 nervures of its wing-cases. 



Pilumnus, Leach, 1815, is represented in Great Britain 

 by the single species Pilumnus hirtellus (Linn.), but fnr 

 the world at large more than eighty forms have b 

 described under separate specific names, and still 

 the discriminating criticism of some future monc 

 In this genus, as at present defined, the antero-laudral 

 margins are normally armed with spines instead of the 

 usual teeth, and the pleon is seven-jointed in both sexes. 

 But when the description and figures of Pilumnus xan- 

 thoides, Krauss, 1843, are examined, they exhibit not 

 spines but rounded teeth or lobes on the antero-lateral 

 border, and a five-jointed pleon in the male. Thus there 

 is primd facie reason to suppose that this species ought to 

 be removed to some other genus. Otherwise the boun- 

 daries of the existing genus must be enlarged, whereas for 

 convenience they rather require to be narrowed. 



Pirimela, Leach, 1815, like Pilumnus, is represented in 

 Great Britain only by a single species, Pirimela denticulata 

 (Montagu), which occurs also in the Mediterranean, but, 

 unlike Pilumnus, it is not represented by any other species 

 elsewhere. In this genus the pleon of the female is seven- 

 jointed, but that of the male five-jointed, the three middle 

 joints being coalesced into a single piece. It differs from 

 all the rest of the Cyclometopa in the character of the third 

 maxillipeds, for here the fourth joint receives the articula- 

 tion of the fifth on its inner instead of on its apical 



margin. 



Family 2 . Trapeziidce. 



'' Carapace depressed and nearly quadrilateral, smooth, 

 with the postero-lateral angles truncated, the dorsal regions 

 not defined ; the antero-lateral margins are straight, form 

 a right angle with the front, and are entire or have but one 

 tooth (the lateral epi-branchial tooth) developed. The 



