Introduction. 
Oc knowledge of the marine Lamellibranchiata of the Gulf of Siam was for 
a long time very limited. Thus, when Professor Ep. v. MARTENS, in 1860, published 
his paper “On the Mollusca of Siam” in the “Proceedings of the Zool. Society 
of London,’ he mentioned in it only 7 species of marine Lamellibranchiata. Nor 
was our knowledge of the fauna enlarged until L. Moruer described Mr. Pavir’s 
collections in the “Journal de Conchyliologie,’ 1889; but in 1891, when Pau 
Fiscuer published his “Catalogue et distribution géograph. d. Mollusques terrestres, 
fluviatiles et marins d’une partie de l’Indo Chine”! he was able to record 72 species 
of marine Lamellibranchiata from the Gulf of Siam, and this number has been in- 
creased to 85 species by Pavir’s new collections, which Crosse & FiscHEr, in 1892, 
described in the “Journal de Conchyliologie.” By Dr. TH. MorTENSEN’s thorough 
and excellent dredgings and collections from the eastern part of the Gulf of Siam 
during the years 1899—1900, the number of known species of Lamellibranchiata 
from that place has now been increased to 379 species. But the fauna is without doubt 
still far from exhausted by this last addition, because the Danish Expedition dredged 
only in the northern and eastern parts of the gulf, and only down to a depth of 
about 85 fathoms. The Mollusc fauna in the western part and also in the deeper 
parts in the middle of the gulf is still unknown. It is true that the “Skeat-Expedi- 
tion” has made collections on the Malacca side of the gulf, but the Mollusca of 
these collections have not yet been worked out. Thus, as it is only the littoral 
fauna of a part of the gulf which is known, it must be understood that the present 
list is only the beginning of a complete list of the species occurring in the gulf. 
Though it may be supposed that the littoral fauna of the northern and eastern 
parts of the gulf is now almost completely known, yet it is probable that the fauna 
of the western side of the Gulf of Siam is richer, and contains many species which 
have not hitherto been taken on the eastern side. This is owing to the fact that 
the water on the Malacca side is much purer and fresher than in the eastern part, 
where the outlets of the numerous rivers make the sea-water foul and muddy and 
also less saline.” 
1 Bulletin Soc. Hist. Nat. d’Autun. IV, 1891, pp. 87— 276. 
2 Cf. TH. Mortensen, “The Danish Expedition to Siam, 1899—1900. Results of the zoological 
Collections.” Kgl. Danske Vidensk. Selsk. Skrifter, 6. R. Natury. og mathem. Afd. XII, 2, 1902, pp. 155—57. 
