The crabs here described are the result of the exploration by Dr. Th. Mortensen 

 of the eastern coast of the Gulf of Siam from Koh si Chang, near the head 

 of the gulf, to Koh Kong at the south, in about 11° north latitude. The passages 

 formed by the numerous islands fringing the mainland proved to be an excellent 

 collecting ground. Operations were continued for a period of about four months 

 (from December, 1899, to March, 1900) and from the shore line to a depth of 30 

 fathoms. The collection obtained is a large one, comprising 204 species, and prob- 

 ably represents a large proportion of the fauna actually existing there. A few 

 specimens- are from Singapore. 



The Gulf of Siam occupies a somewhat central position in the marine Asiatic 

 fauna, being midway between the Chinese and Japanese seas on the one hand and 

 the Indian Ocean on the other, while in an intermediate direction radiates the 

 Malayan Archipelago. While the Brachyura are in general Indo-Pacific in character, 

 72 per cent of the previously known species being enumerated by Alcock in his 

 "Materials for a Carcinological Fauna of India," yet there are certain features 

 which seem to be peculiar to the Gulf of Siam. Of the twenty-seven new species, 

 three represent as many new genera, and one makes necessary a new subfamily. 

 Several of these forms are catometopous crabs with a semicircular carapace, that 

 is, with a squarish posterior, and an arcuate anterior outline, the antero-lateral 

 angles being rounded off. The collection is noticeably rich in species of this build, 

 belonging to the families Gonoplacidœ (subfamilies Rhizopinœ and Hexapodinœ) and 

 Pinnotheridœ. Next in importance are the Leacosiidœ which number 31 species. 



One is struck with the many minute specimens in the collection; their pre- 

 sence may be attributed either to the zeal of the collector or to their relative 

 abundance in this sheltered arm of the sea. 



The classification used in this paper is that of Borradaile (cf. Ann. Mag. 

 Nat. Hist. (7), XIX, June, 1907, 457—486). In the case of species included in 

 Alcock's memoir above mentioned,' synonymicai references are restricted to that 

 work, which contains full synonymy and descriptions. 



The types of new species are in the Copenhagen Museum and a set of cotypes 

 is in the United States National Museum. 



■ Journ. Asiat. Soc. Bengal, LXIV, 1895; LXV, 18%; LXVII, 1898; LXVIII, 1899; LXIX, 1900. 



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