Five double crochet shell stitch
US shells spread five dc from one insertion point and are anchored by sc.
Counted practice swatch
Chain 26, a multiple of 6 plus 2.
Work the repeat
Row 1: sc in second chain; [skip 2 ch, 5 dc in next ch, skip 2 ch, sc in next ch] 4 times. Row 2: ch3 (dc), 2 dc in first sc; [sc in third dc of next shell, 5 dc in next sc] 3 times; sc in third dc of final shell; 3 dc in last sc. Row 3: ch1, sc in first dc; [5 dc in next sc, sc in third dc of next shell] 3 times; 5 dc in final sc, sc in top of ch3. Repeat rows 2–3.
Read the fabric
Full shells have five posts; each half shell at an edge has three. Anchor into the middle post so the next row stays centered.