These are egg cases of Cassis madagascariensis.
I thank my friend and underwater photographer, Tracey Winholt who has kindly shared the following photos.
These photos are valuable, because I found no other photos of eggs of this species on the internet except of black-and-white photos by D’Asaro. They are also valuable because this large species is rare in some areas. For example, it is rare in Florida.
Egg cases of Cassis madagascariensis were described for the first time in 1969 as the first described egg cases of the genus Cassis.
The locality is Cozumel, Mexico. It is an island of the Caribbean Sea, nearby the Yucatán Peninsula. The habitat is sea grass beds in the depth 7-8 meters. These egg cases are not uncommon in the grassy areas in Cozumel.
The another egg case was also laid on an algae. It is some fan shaped green alga of the genus Avrainvillea, maybe Avrainvillea erecta. Eggs were laid to the alga to both of its sides:
Each egg capsule is quadrangular as viewed from the top. Egg capsules are irregularly arranged on the substrate. Egg capsules with developing eggs are light brown in color.
D’Asaro counted 2400 embryos in one egg case on average. There are developing embryos on the photo:
Maybe some of those embryos will grow up into an adult snail with a shell length up to 30 cm or 40 cm:
D’Asaro C. N. 1969: The spawn of the emperor helmet shell, Cassis madagascariensis Lamarck, from South Florida. Bulletin of marine Science, 19(4): 905-910.
Sept J. D. 2016: Atlantic Seashore Field Guide: Florida to Canada. Rowman & Littlefield. page: not numbered. (Note that there should be “cm” instead of “mm” in the size section.)