Mr. R. I. Pocock on Isometrus americanus (Linn.). 59 
Chela.—Upper surface of humerus covered with very fine 
close-set granules and bounded before and behind by a con- 
spicuous series of larger granules ; anterior surface bounded 
below by a similar series and completely divided into an 
upper and a lower half by a coarser series parallel to the last- 
mentioned series ; inferior surface smooth ; posterior surface 
furnished with one series; the whole segment therefore is 
furnished with five parallel series of granules. The brachium 
furnished with seven keels—two in front, two above, two be- 
hind, and one below, all granular except the last named, 
which is smooth; the intercarinal spaces finely shagreened. 
Hand a little wider than brachium, keeled; three keels running 
from the immovable finger to the proximal end of the seg- 
ment, one bounding the ‘ hand-back ”’ above and two shorter, 
but unequal, keels running obliquely from the proximal 
end of the hand towards the movable finger. Fingers long, 
incurved, almost in contact when closed; movable finger 
longer than brachium, furnished with a conspicuous lobe, 
which fits behind a corresponding but smaller lobe on the 
immovable finger. 
Legs furnished with granular keels. 
Pectines furnished with twenty-three teeth; the proximal 
intermediate lamella produced into a large, rounded, smooth 
lobe. 
Measurementsin millimetres.—Length of cephalothorax 114, 
width 124; distance of eyes from posterior margin 7}; length 
of tail 67; length of first segment 8, width 64, height 64 ; 
ditto of second 10, 63, 6; ditto of fifth 104, 64,65 ditto of 
vesicle 6, 64, 525; length of aculeus 5; length of humerus 
121, width 34; ditto of brachium 13, 43; length of “ hand- 
back” 9, width of hand 5}; length of movable finger 143, of 
pecten 73. 
Several female specimens collected in the island of Santa 
Lucia for the West-Indian Exploration Committee by Mr. 
G. A. Ramage. 
This species is very closely allied to Isometrus americanus 
(Linn.), but may be distinguished by the absence of a spine 
under the sting and by its greater number of pectinal teeth. 
‘The male is unknown. 
