Bei i 
CRUSTACEA OF THE MERGUI ARCHIPELAGO. 285 
Two adult specimens were collected in the Mergui Archipelago. 
They measure about 24 centim. from the tip of the rostrum to the 
end of the terminal segment of the postabdomen. Mr. Spence 
Bate, in his critical examination of the Peneide (Ann. & 
Mag. Nat. Hist., 1881, vol. viii. p. 178), comes to the conclusion 
that this species and P. indicus, M.-Hidw., are mere varieties of 
P.monodon, Fabr. This may be so; but it appears to me even 
more probable that P. semisulcatus, de Haan, is identical with 
P.monodon, Fabricius, because the diagnosis of the latter agrees 
perfectly with that of the former ; but P. semisulcatus 1s probably 
distinct from P. indicus, the typical specimen of which has been 
figured by Spence Bate, though both species are apparently closely 
allied. P. indicus probably differs from the species of the ‘ Fauna 
Japonica’ by the following characters :—Although the rostrum 
presents the same form in both species, being slightly elevated at 
the extremity, it is constantly armed in P. semisulcatus with three 
teeth at its lower margin, but in P. indicus with four or five; and 
the posterior tooth is placed a little more backwards in regard to 
the hepatic spine in the former than in the latter. In the 
second place, | may observe that the lateral sides of the cepha- 
lothorax of P. semisulcatus present a horizontal crest, close 
to and immediately below the deep hepatic sulcus (described for 
the first time by Hilgendorf), proceeding towards the antennal 
scales, which does not seem to occur in P. indicus, according to the 
figure of this species given by Spence Bate. I may also add that in 
P. semisulcatus the antennal sulcus* is very deep in its posterior 
portion, as is also the hepatic sulcus, but the gastrohepatic sulcus is 
faintly defined, as has been already pointed out by Miers. The 
third postabdominal segment is not keeled in either of these speci- 
mens, and the median dorsal keel of the fourth segment does not 
even extend to its anterior margin; I therefore presume that 
Mr. Miers is wrong in describing the third to the sixth segment 
as keeled. 
The rostrum in both specimens is §- and £-dentate. I may add 
that the flagella of the internal antenne have both nearly the same 
length, being a little longer than the peduncle, 7. e. the distance 
from the distal end of the terminal joint of the peduncle to the 
anterior margin of the carapace. ‘he upper or external flagellum 
is a little broad and grooved along the proximal third of its 
* T here follow the terminology of Stimpson. 
