We discuss the morphology, histology, ecology, distribution, systematics, and evolutionary relationships of "Lithoglyphopsis" aperta Temcharoen, the snail host of Mekong River Schistosoma sp., and part of a vast, complex, endemic hydrobid fauna consisting of 11 genera and over 80 species. "L" aperta is a member of the Hydrobiidae (as broadly outlined by Fretter & Graham, 1962), subfamily Triculinae (as defined by Davis, 1968b). "L" aperta cannot be assigned to Ltihoglyphopsis because its shell and radula differ from those of the type-species, L. modesta (Gredler) from China, and because L. modesta is apparently more closely allied to other Mekong River genera in these traits. The female reproductive system of "L." aperta is similar to that of Tricula burchi Davis, a species from NW Thailand outside the Mekong River drainage. It is not possible to assign aperta to a named genus until the morphologies of numerous other hydrobiid taxa in the Mekong River are unknown. "L." aperta is typically hydrobiid in grade of morphological organization, in the nervous digestive, ctenidial and male reproductive systems. Differences from other hydrobiid taxa are in the female reproductive system and micromorphological features of the digestive tract. "L." aperta and species of Tricula from Thailand have a female reproduction system where sperm enter at the posterior end of the mantle cavity and travel to the bursa copulatrix via a spermathecal duct. These and related traits are the basis for the subfamily Triculinae. Hydrobiid taxa from Europe (Hydrobiinae s.s.) belong in a different phyletic line, where sperm enter the female reproductive system at the anterior end of the mantle cavity and travel via a ciliated groove in the palial oviduct to the bursa copulatrix. "L." aperta, as well as taxa of the Pomatiopsinae (e.g. Oncomelania, Pomatiopsis), differ from most known mesogastropods in lacking a hypobranchial gland. "L." aperta, other triculines, pomatiopsines and hydrobiines, as well as taxa studied in the Bithyniidae, Truncatellidae and Assimineidae differ from other mesogastropods, e.g. Viviparidae, Pleuroceridae, Littorinidae, etc., in that the salivary glands are dorsal to the nerve ring i.e. do not pass through the nerve ring. "L." aperta lives on solid substrata, particularly wood, shells and leaves in the Mekong River from Khemarat, Thailand, to the Cambodian border, 200 river miles downstream. The range probably extends another 100 river miles downstream to Kratie, Cambodia. It is an "r"-selected species by having a high density-independent mortality and using much of its resources for reproduction. The species is a colonizer in a river with severe annual floods. Females live less than 12 months; they apparently lay eggs in late January or February and die. In early March neither adults nor young can be collected. By mid or late March young suddenly flourish. The new generation does not mature until late May or June, after the beginning of the rainy season...