These must be quite common. Me and my siblings always used to say they were jellyfish eggs and throw them at each other. Dead jellyfish makes it a bit grosser.
I have just come back from Menorca in the Balearic islands in the western Mediterranean, and we couldn't sit along the beach for the smell of dead jelly fish, they were rather small and pinkie brown colour and were rotting along the strand line and in the water. In one inlet they were swimming about but the place still stank, due to the dead ones around them.
Q
Quoll
Moon shell egg masses
Those half-moon jellies are the egg masses of an intertidal gastropod, Conuber (or Polinices) sordidus. The common name is nearly as exciting - the Leaden Moon shell.
The snail isn't real big, but the eggs are laid embedded in a protein matrix which swells up when exposed to sea water.
