Proserpina amazoniensis

Here another, recently extinct, false river serpent which, how the name already suggests, was a inhabitant of the amazonian basin but also the Orinoco. 
We don't know much about this species and since 1943 no one has seen one again, some cryptozoologists (yes, even the Institute of supposed fictional biology have some) that the species might still strive through some, still bad known, regions of south america, but there is no prove yet, beside some local tales which might just describe big caimans. 
The morphology of this species is mostly known because of some subfossil finds and old depictions of early explorers. The behavior is mostly unknown but Proserpina seems to have been mostly a piscivorous predator and when you look through the fish-fauna of the Amazon this isn't surprising. 
A recent revision of the taxon showed that their could be to subspecies or maybe even species, Carter et al. 2013 named P. a. major and P. a. wagleri and suggested to
name material from older sediments as P. incurva, a sight which is still matter of controversial discussions.
To the Indians of the amazon this creature is known as Yacumama.

Trivia: one of the first who described this species in detail was Alexander von Humboldt during his journey on the Orinoco.

Other extant and extinct natrixosaurs:

Halidraco: hyrotrioskjan.deviantart.com/a…
extinct freshwater sea serpents: hyrotrioskjan.deviantart.com/a…
Cavinatrix: hyrotrioskjan.deviantart.com/a…
Gracilinatrix: hyrotrioskjan.deviantart.com/a…
Bibonatrix: hyrotrioskjan.deviantart.com/a…
Asperognathus: hyrotrioskjan.deviantart.com/a…
Natrixosaurus: hyrotrioskjan.deviantart.com/a…