(μέγας κόλπος/mégas kólpos, Ptol. 7,2,1; 7,3,1). A large ocean gulf in India extra Gangem, adjoining the Sinae, i.e. south-east Asia, with three rivers: Daonas, Dorias and Seros (Ptol. 7,2,7). Although the geography of south-east Asia seems hopelessly distorted in Ptolemy, and all interpretations of place names in that region must remain highly hypothetical, nonetheless the Magnus Sinus can be identified with the waters lying between the Malacca Peninsula and southern China.
Bibliography
H. Treidler, s.v. Μέγας κόλπος, RE Suppl…