I'd highly doubt that these are actually Euspira lewisii. (now classified as "Lunatia lewisii")

Lunatia lewisii's egg casing are at least 6" across.
The actual snail gets larger then you fist.
As well, its mantle comes up and covers half of its shell (like a cowrie)
so it is visually unique.

What you have definitely looks like a an egg case, and may have come from your new snails.

But I highly doubt they are Lunatia lewisii.