Moon snail egg casing (Naticidae)

Madidir III, Lembeh.
Without the progenitors nearby is quite hard to tell the species :-)
The first time I saw moon snail eggs was in New England, US and thought they were flat tires :-D

http://www.wildsingapore.com/wildfacts/mollusca/gastropoda/naticidae/naticidae.htm
http://www.seaslugforum.net/find/naticid

In this link you can see the moon snails from New England US area with their egg case:
http://www.andrewjmartinez.com/image.pl/400/_Northern_Moon_Snail_with_its_egg_case.html

In this link you have a cool drawing on how the female makes this egg casing!
https://www.amusingplanet.com/2017/06/the-sand-collars-of-moon-snail.html?m=1

Sand collar
https://en.wikipedia.org/wiki/Sand_collar
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Sand collar from the moonsnail Neverita josephinia and shell of that species on the right, Mediterranean Sea
A somewhat damaged sand collar of Euspira catena. When the light is shing through the collar, it is possible to make out the individual egg capsules within it.

Sand collars are the characteristic egg masses of one family of sea snails, the moon snails, marine gastropod mollusks in the family Naticidae. These egg masses are often found washed up either whole, or sometimes in fragments, on sandy beaches where moon snails are living, either intertidally or subtidally.
Description

When they are intact, sand collars are shaped rather like an old-fashioned detachable shirt or blouse collar (hence the name). The sand collar consists of sand grains cemented together by a gelatinous matrix, with the embedded eggs contained within the matrix. The collar is laid by the female moon snail, and the size of the sand collar gives an indication of the size of the adult female moon snail that laid it; larger species of moon snail lay larger sand collars.

A fresh sand collar feels stiff and yet flexible, as if it were made out of plastic. Each sand collar contains thousands of capsules, each one housing one or several live embryos. In species with planktonic development, these embryos hatch out as bilobed veligers. After the eggs hatch, the sand collar disintegrates.