[split] BAV Pal. lat. 291. De rerum naturis. 1425

[Linda]

I interpret the one on the right here as a concave niche (similar to the one on the left) decorated with a shell. 



The motif could come from classical nymphaei and was later adopted by Renaissance and Baroque art. Likely, classical examples could still be seen in Europe in the middle ages. It is possible that the motif also appears in De Balneis, but I didn't check.

Source of the Tiber



I dont know if this is the same thing as the nymphaei but there often seems to be architecture built around springs.