A shellfish monster that appears in the Gazu Hyakki Tsurezure Bukuro. Sekien gives the following explanation:

雀海中に入てはまぐりとなり、田鼠化して鶉となるためしもあれば、造化のなすところ、
さゞえも鬼になるまじきものにもあらずと、夢心にもおもひぬ。
If a sparrow enters the sea and becomes a clam, and the field-rat transforms into a quail, then in the workings of Creation, it is no impossible thing that a turban snail might become a demon, I saw it not in my dreaming mind.

The sparrow which becomes a clam and the field-rat which becomes a quail are images taken from a proverb in the Yue Ling (a chapter of the Chinese Classic of Rites), which alludes to the wonders found in an uncanny universe. Sekien seems to have had similar feelings about the notion of a turban snail transforming into a monster.

A similar image appears in a Hyakki Yakō picture scroll in the archives of the Tokyo National Museum. Here a turban snail monster dashes along with the the demonic parade, dragging a robust, laughing child that has grown out of a clamshell, and glancing back at a Buddhist scepter which has transformed into a dragonfly.

Citations
Inada p. 278, Murakami 2005 p. 152, Yumoto 2005 A p. 62.