Mr. R. I. Pocock on Isonietrus americaiius {Linn.). 59 



Chela. — Upper surface of liumerus 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. 



Measurements in miUhnetres. — Length of cephalothoraxll^, 

 width 12^ ; distance of eyes from posterior margin 7g- ; length 

 of tail 67 ; length of first segment 8, width 6^, height 6^ ; 

 ditto of second 10, 6|, 6 ; ditto of fifth lOf , 6^, 6 ; ditto of 

 vesicle 6, 6^, 5f ; length of aculeus 5 ; length of humerus 

 12|, width '6\ ; ditto of brachium 13, 4f ; length of " hand- 

 back " 9, width of hand b\ ; length of movable finger 14f , of 

 pecten 7|. 



Several female specimens collected in the island of Santa 

 Lucia for the West-Indian Exploration Committee by Mr, 

 G. A. liamage. 



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 pcctinal teeth. 

 The male is unknown. 



