What are these dotted ball shaped things I circled in yellow in the image?
I thought they may be sea urchin skeletons. Though the things in the image may be too many and too large?
They are in a mural in Brighton, England. The subject is "Protecting Costa Rican Wildlife", so I guess they are of some native species there.
Urchin shells
To be more specific on the terminology they are sea urchin tests: https://en.wikipedia.org/wiki/Test_(biology)
Yes, missing the spines.