I’ve never heard that Turritella Agate was petrified fish eggs but
it definitely consists of fossilized (agatized) snail shells.
However, as many experts have pointed out, they’re not turritella:
“Turritella agate, Wyoming, dark brown to black agate with generally
white shells of gastropod Goniobasis misidentified as Turitella=85”
(Credit Geological Society of America and agatelexiconletter).
But the incorrect name is so well established that well-known
lapidary writer June Culp Zeitner, along with other experts,
recommended long ago that the “turritella” name be preserved. The
spelling with two “r’s” seems to be preferred by paleontologists.