The triangular kidneyshell, scientific name Ptychobranchus greenii, is a species of freshwater mussel, in the family Unionidae, the river mussels. It is endemic to Alabama in the United States, where it is known from several rivers and streams in the Mobile River Basin. It is a federally listed endangered species of the United States.This aquatic bivalve mollusc is somewhat oval in shape and may reach 10 centimeters in length. It is yellow to yellow-brown in color.