cockleshell

plural cockleshells

1. The shell of a cockle (or similar shell).

   1 more example

   The moment land had disappeared, the great canoe seemed no larger than a cockleshell; the waves lapped dangerously near to the tops of the gunwales, all but toppling into the canoe; and that is the curious thing about Polynesian canoes: the water always laps just to the tops of the gunwales, no matter whether the canoe is shallow or deep, whether empty or heavily laden.

   1929, Robert Dean Frisbee, The Book of Puka-Puka, Eland, published 2019, page 203

2. A small, flimsy boat.