Medieval sirens and mermaids

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Of course we can assume a different intention, but what's on the page is a Jonah type.

A You are not allowed to view links. Register or Login to view.by Ellie Velinska also shows interesting similarities with hellmouth iconography. 
As I mentioned in the comments there, the fish-looking monster whose mouth is often depicted as the gate to hell is the Leviathan.
Ellie makes an interesting parallel with the purgatory page in the Tres Riches Heures... (Condé Ms 65) and especially BL Royal 19 C I - Last judgement