Those are egg capsules and embryos of a marine snail Tarantinaea lignaria from the family Fasciolariidae. Its synonyms are Pleuroploca lignaria and Fasciolaria lignaria.
Egg capsules of Fasciolariidae are possible to identify to the genus level and to the species level. But majority of egg capsules of Fasciolariidae is not known.
The egg capsules looks like on these photos. There is a circular opening in the middle of the cup. Newly hatched snails are escaping through the opening. There are visible embryos through the transparent wall of the capsules and there can be some few embryos inside one capsule. Exact number of embryos in one capsule was not published. It seems that this species may have a direct development.
These egg capsules comes from the southern Croatia. I have taken these photos at September 28, 2016. Majority of snails hatched out already, but there are still some (live?) embryos visible in few capsules. There is also a juvenile snail on one of the capsules that probably (but not surely) belong to the same species.
Detail cropped from the previous photo shows a juvenile snail:
Another view of the cluster of egg capsules:
Detail cropped from the previous photo shows two egg capsules:
The drawing of egg capsules of Tarantinaea lignaria is in the series on Gunnar Thorson’s collection of prosobranch egg capsules. I did not made proper measurements.
I thank to Dr. Paolo Russo from Italy for the species identification.
Gunnar Thorson (1906-1971) was professor of marine biology at the University of Copenhagen. He assembled the collection of egg capsules but he did not publish results of the whole collection by himself. Series of four monographs based on Thorson’s collection was published in 1992-2000 and it is very useful resource for anybody interested in this field.
D’Asaro C. N. 2000: Gunnar Thorson’s world-wide collection of prosobranch egg capsules: Fasciolariidae. Ophelia: 52(2), 77-112. DOI: 10.1080/00785236.1999.10409421