Dichotomous keys are the best way to identify just about anything you can find in the natural world. If you've never used one before, and are unsure exactly how it works, here is a very good explanation: http://nerds.unl.edu/Pages/preser/sec/skills/dkeys.html

1a. Disc rhomboid or heart-shaped .................... Family Dasyatidae

1b. Disc circular or oval .................... Family Potamotrygonidae

KEY TO THE FAMILY POTAMOTRYGONIDAE (River Stingrays)

1a. Disc circular, relatively large compared with the tail, and slightly concave along the front. Knob-like structure on outside of spiracles .................... Paratrygon aiereba

1b. Disc oval, no knob-like structures on spiracles.................... 2

2a. Tail very long, more than twice the length of the disc. Eyes very small. Ventral finfold present at end of the tail .................... Plesiotrygon iwamae

2b. Tail shorter, less than twice the length of the disc. Eyes large. Both dorsal and ventral finfolds present at the end of the tail .................... Potamotrygon

KEY TO THE GENUS POTAMOTRYGON

1a. Large, pronounced spots or ocelli present on top of disc ................. 2

1b. Top of disc without large spots .................... 5

2a. Spots irregularly shaped and rust-colored, one row of dorsal tail spines .................... Potamotrygon ocellata

2b. Spots circular or lunate..................... 3

3a. Base color of disc tan to brown, spots yellow to orange and encircled with black ring .................... Potamotrygon motoro

3b. Base color black to very dark greyish brown, spots white or yellowish white .................... 4

4a. Underbelly greyish or speckled, spots most often limited to top of disc, three rows of dorsal tail spines .................... Potamotrygon leopoldi

4b. Underbelly often black, with spots continuing to the underside of disc and onto the tail .................... Potamotrygon henlei

5a.

KEY TO THE FAMILY DASYATIDAE (Stingrays)