Also called CLAMSHELL (Ruth E. Finley's OLD PATCHWORK QUILTS, 1929, pl. 26). See also this version of SHELL(S) outlined on a full grid, along with instructions on how to draft your own patterns, in the QUILTING AND PATCHWORK DICTIONARY (p.214) by Rhoda Ochser Goldberg, 1988. As a continuous design (flipped top to bottom), it is called SHELL CHAIN and dates back to the Ladies Art Company Catalog, #52, published in 1898, according to Barbara Brackman's ENCYCLOPEDIA OF PIECED QUILT PATTERNS (#181b). Jinny Beyer's QUILT ALBUM suggests an unexpected and interesting association of the SHELL(S) tiling pattern with TUMBLERS (p. 433-3).
SHELLS is also a traditional Japanese pattern (seigaiha, "blue ocean wave," illustrated left, also flipped). In kimono patterns, it is combined with classic hexagon stars, as featured in JAPANESE DESIGN THROUGH TEXTILE PATTERNS, by Frances Blakemore (p.167). Other aquatic blocks, at this site, include:
TOAD IN A PUDDLE
DUCK CREEK PUZZLE