CBUSTACEA OF THE MEEauI ARCHIPELAGO. 153 



millim. 



Length of the cephalo thorax 9 



Distance between the external orbital angles 10| 



Distance between the internal orbital angles 4| 



Length of the larger hand 7|- 



G-enus Metaplax, M.-Mdw. 

 (Syn. : Rhaconotus, Gerstaecker, 1856.) 



The genus Metaplax was established in 1852 by the late 

 H. Milne-Edwards for two small Brachyura from the Indian 

 Ocean. This celebrated carcinologist referred it to his section 

 G-onoplaciens, and considered it as making a transition between 

 the genera Macrophthalmus, Gonoplax, and Helice. Tour years 

 afterwards, G-erstaecker described an interesting new form of 

 Sesarmacea under the name of JRliaconotus crenulatus. Sur- 

 prised by the extraordinary length of the legs, he was unable to 

 refer his new species to any one of the genera of Sesarmacea 

 enumerated by Milne-Edwards, and not observing its close afi&nity 

 to the genus Metaplax, he founded a new genus Rhaconotus. 



In 1858 Stimpsou published a new species of Metaplax, dis- 

 covered at Hongkong. 



Inl865, Heller, the author of the Report on the Crustacea col- 

 lected during the Novara Expedition, described a young female 

 specimen of Brachyura from Ceylon under the name of Helice 

 dentipes. 



The Mergui Archipelago contains not only representatives 

 of one of the two species of Metaplax described in 1852, but 

 even two new species of this group, and moreover a fine series 

 of specimens of the rare Rhaconotus crenulatus and of Heller's 

 Selice dentipes. 



A careful examination of these five species has led me to the 

 conclusion that they are all closely allied to one another, and 

 ought to be referred to the same genus, and that this genus is 

 most closely allied to Helice, de Haan. 



"When comparing them with one another, and with the typical 

 representative of Helice, the Japanese Helice tridens, de Haan, 

 it is not difiicult to observe that these species are all closely 

 allied, or even agree with one another as regards the structure 

 of the cephalothorax, that the very different appearance of 

 Rhaconotus cremdatus and Helice tridens must be chiefly ascribed 

 to the thicker cephalothorax of the latter and to the different 



