The main purpose Of this work is to make known, by 'means of a comprehensive and not too technical account, the nature, extent, and richness of the fish fauna inhabiting the fresh waters Of the ancient Kingdom of Siam; to describe the relationships Of the various groups; and to indicate what further collecting Of specimens is necessary or likely to prove profitable scientifically. It is the intention to take cognizance herein of every species of fresh-water fish known from or ascribed to Thailand. In cases of reasonable doubt as to the validity of species described from the country Or as to the accuracy Of local records or citations, it has been thought proper to admit them to the catalog and trust to future Observations to establish the facts.