Notes on the non-marine molluscs of the island of Borneo 6. The genus Opisthostoma (Gastropoda Prosobranchia: Diplommatinidae), part 2

The species of the genus Opisthostoma (Diplommatinidae) occurring on Borneo are revised. In total, 67 species and 6 subspecies are included; 36 species and 4 subspecies are new. Contrary to the opinion, expressed in Vermeulen (1991), that Plectostoma should be regarded as a genus, it is now argued that it is best included in Opisthostoma. A new key to all the species is presented, but the descriptions of the species treated in Vermeulen (1991) are not repeated. All species except O. javanicum are endemic to Borneo. In its present circumscription, the genus Opisthostoma is presumably monophyletic on account of a unique character: the inverted last half whorl of the shell.