Here is the full list of all 10 Seashell achievements.
Log a legendary Nautilus shell
Fill a page with drawings
Achieve a level 10 research badge
Log a legendary Abalone shell
Discover all the description fragments for a single shell
Return your first collection to the sea
Discover all the description fragments for every shell
Log a legendary Carrier shell
Log 10 of the same species of shell in a single session
Log 12 different species of shell in a single session