In 1959, Mr. A. Hoogerwerf obtained a number of small Mollusca from the gizzard of a godwit (Limosa spec.) shot near Koerik, West Irian, New Guinea, that eventually came into my hands for identification. Though the specimens clearly belonged to a species of the genus Mesodesma (Mactracea, fam. Mesodesmatidae), further identification with any of the known species of this family proved to be impossible. This stimulated me to revise the Mesodesmatidae. During this revision many species were encountered, that had each been described under several different names. New synonymies in the genera Ervilia Turton, 1822, and Coecella 1) Gray, 1853, will be dealt with in the present paper. Diagnosis of the Mesodesmatidae. Shell equivalved, small to moderately large (maximum length 3-140 mm) and of ovate to triangular, mostly inequilateral shape. The umbones are generally on the posterior side. The external ligament is short and weak, but there is a stout resilium fitted into a deep resilifer. The hinge is rather solid. Each valve has one cardinal. On the left valve there is one lateral on each side of the umbo fitting between the two opposite laterals of the right valve. The pallial sinus is variously developed, or even absent in some genera. Diagnosis of the genus Mesodesma 1) Deshayes, 1832. The maximum sizes of the shells are very different from one species to another. The umbo is generally on the posterior side, rarely in the middle but never on the anterior side. The shells are white with a yellow to brown periostracum which is often worn off, partly or even completely. The resilifer never points to the posterior side. The palliai sinus is always present.