4 
CRUSTACEA OF THE MERGUI ARCHIPELAGO. 109 
presents some varieties, which it will be useful to record. The 
rich materials at my disposal have enabled me to study the slight 
variations which are presented by the cephalothorax and by the 
larger hand of the male; and I have compared them with a 
typical specimen of this species from the Paris Museum, so that 
my identification may be accepted as correct. 
Gelasimus Dussumiert belongs to that section of the genus 
which is characterized by a narrow front between the eyes; but 
it may be distinguished from the other species of the section 
chiefly by the shape of its cephalothorax and by the form of the 
larger hand of the male. 
The cephalothorax of the adult male (Pl. VII. fig. 2) has a 
smooth and bright upper surface, which is very arcuate and 
convex longitudinally, and for the shape of which I refer to my 
figure. The front, which has been well figured by Hilgendorf 
(J. c. fig. 16), is very narrow and constricted between the inser- 
tion of the eye-peduncles, and is again enlarged a little below at 
the rounded anterior margin, where it presents a. minute, tri- 
angular, median incision. The median furrow extends a little 
beyond the middle of the front and is very narrow and linear ; 
between the insertion of the eye-peduncles its breadth measures 
a little Jess than a third of that of the front, so that the raised 
margins, which border the furrow on each side, appear a little 
broader than the furrow itself, which is nearly equally broad along 
its whole length. In very young males, in which the distance 
between the external orbital angles measures 14 millim., the 
median frontal furrow is a little broader, so that its breadth 
somewhat surpasses that of the lateral margins. These raised 
lateral margins of the front pass laterally into the sinuated upper 
margin of the orbits. The latter is bordered below by an 
accessory line at a short distance from it, running parallel to 
the upper margin nearly from the spot where the thickened 
basal portions of the eye-peduncles pass into the slender stalk, 
near to the point where the latter passes into the cornea. The 
upper wall of the orbits therefore presents a long narrow stripe 
between the upper margin and that accessory line; this stripe 
appears a little broader in the female than in the male, the 
distance between the upper orbital margin and the accessory line 
being a little shorter in the male than in the female. 
The external orbital angles are very acute and directed 
