64 A HISTORY OF RECENT CRUSTACEA 
wrist of the chelipeds has a long sharp crest, and the 
rapid rubbing of this crest against the striz 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 for 
the world at large more than eighty forms have been 
described under separate specific names, and still await 
the discriminating criticism of some future monograph. 
In this genus, as at present defined, the antero-lateral 
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 prima 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 Pilwmnus, 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.—Trapezude. 
‘Carapace depressed and nearly quadrilateral, smooth, 
with the postero-lateral angles truncated, the dorsal regions 
not defined; the antero-lateral margins are straight, form 
aright angle with the front, and are entire or have but one 
tooth (the lateral epi-branchial tooth) developed. The 
