The ornamental design of a clamshell-shaped floatation device, as shown and described.
FIG. 1 is a top, front, right-side perspective view of the clamshell-shaped floatation device.
FIG. 2 is a front view.
FIG. 3 is a rear view.
FIG. 4 is a right-side view.
FIG. 5 is a left-side view.
FIG. 6 is a top view; and,
FIG. 7 is a bottom view.
The design features a clam-shell shaped flotation device for use in bodies of water. The broken lines showing the inflation points for the clamshell-shaped floatation device as seen in FIGS. 3, 5, and 7 form no part of the claimed design.