CRUSTACEA OF THE MERGUI ARCHIPELAGO. 153 
millim. 
Length of the cephalothorax..... pete te neste 5 take ook 9 
Distance between the external orbital angles...... 104 
Distance between the internal orbital angles ...... 42 
Menothvor tie larger hand:....-...5.+.s.s.e1ess 2 
Genus Merarnax, I0-EHdw. 
(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 
Gonoplaciens, and considered it as making a transition between 
the genera Macrophthalmus, Gonoplax, and Helice. Four years 
afterwards, Gerstaecker described an interesting new form of 
Sesarmacea under the name of Rhaconotus 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 affinity 
to the genus Metaplax, he founded a new genus Rhaconotus. 
In 1858 Stimpson published a new species of Metaplax, dis- 
covered at Hongkong. 
In1865, 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 
Helice 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 difficult 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 crenulatus and Helice tridens must be chiefly ascribed 
to the thicker cephalothorax of the latter and to the different 
