CKUSTACEA OF THE MERGFI AKCHIPELAGO. 285 



Two adult specimens were collected in the Mergui Archipelago. 

 Thej 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 PenaeidiB (Ann. & 

 Mag. Nat. Hist., 1881, yoI. viii. p. 178), comes to the conclusion 

 that this species and P. indicus, M.-Edw., are mere varieties of 

 P. monodon, Fabr. This may be so ; but it appears to me even 

 more probable that P. semisulcatws, de Haan, is identical with 

 P. monodon, Fabricius, because the diagnosis of the latter agrees 

 perfectly with that of the former ; but P. semisulcatus is 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, I 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 f- and |-dentate. I may add 

 that the flagella of the internal antennae have both nearly the same 

 length, being a little longer than the peduncle, i. e. the distance 

 from the distal end of the terminal joint of the peduncle to the 

 anterior margin of the carapace. The upper or external flagellum 

 is a little broad and grooved along the proximal third of its 



* I here follow the terminology of Stimpson. 



