USNM 85895. Rio Salado, Nueva Leon, Mexico. Berlandier! (type)
Popenaias popeii is the only species of the genus to be found as far north as the United States. The other two species of the genus are known from Mexico and further south in Mesoamerica. In general, Central American species in the Unionidae are currently treated as belonging to genera endemic (13) to that area, but Popenaias is one of the few (5) genera to straddle the Nearctic and Neotropical regions. Other Mesoamerican species have closer affinities to South America and are classified in the family Mycetopodidae.
Popenaias is generally similar (shells and soft-parts) to Elliptio, but the former has a distinct breeding period, spawning in the boreal winter rather than the summer. Winter breeding among members of the Pleurobemini caused quite a problem for early classifications that relied heavily on the "brooding period" as a taxonomic character.
