Apalone spinifera emoryi
Hidalgo Co., Texas. February 22, 1994.
These are common turtles in the lower Rio Grande in south Texas. For a number of years I visited Santa Ana NWR, which borders the river, and every visit I could see a number of these turtles, swimming just below the surface near the riverbank in places where the willows hang out over the water.
HerpMapper records for this species are here.