Animal: Azdyryths
Close Surface Cousin: Ichthyosaurs
Source: At The Earth’s Core , Chapter 4
Environment: Water / Ocean
Dian told me they were tandorazes, or tandors of the sea, and that the other, and more fearsome reptiles, which occasionally rose from the deep to do battle with them, were azdyryths, or sea-dyryths—Perry called them Ichthyosaurs. They resembled a whale with the head of an alligator.
Image is a little more pre-1914 than I would like; but seems to fit the conception of what it looked like around ERB’s time.