Anatomy and systematics of Anticorbula fluviatilis (H. Adams, 1860) (Bivalvia : Lyonsiidae) from the Amazon Basin, Brazil and Peru

Anticorbula fluviatilis (H. Adams, 1860) is an enigmatic freshwater bivalve not closely related to the-usual groups of freshwater bivalves. Due to its rarity and consequent lack of specimens, the validity of the genus and spe cies, their occurrence in the freshwater habitat, and even their existence, have been questioned. Other species of closely similar taxa are common in estuarine areas of northern and western South America. The familial allocat ion of Anticorbula has also been problematic. Specimens of A. fluviatilis ( with soft parts) collected in the Amazon Basin including material from the type locality were examined, and the validity of the genus and species conf irmed. A detailed description of the anatomy is provided as the foundation for future systematic comparisons. Diagnostic characters include possibly v estigial lithodesma, small foot with byssus, mantle lobes almost entirely f used (with pedal and pallial apertures only), lack of siphons, very large p osterior retractor muscles of foot, stomach with ducts to digestive diverti cula only on right side, and style sac separated from intestine. The presen ce of these characters suggests allocation within the family Lyonsiidae, bu t this familial allocation should remain provisional until a better definit ion of the Lyonsiidae is made available.