I made some progress with the shells. The way I dye them is in fact shibori in miniature. Here's one piece with threads just unwound showing how the stripes underneath resisted the dye.
This is what they look like with the pleats spread out but without any smocking yet.
It seems that what started as shells may be moving in the more general direction of patterns suggesting fur and feather.
It's beginning to remind me a lot of an arashi shibori scarf I made several years ago in a design I used to call 'feathers'. It was mottled with irregular lines dyed across the pleated texture.
Now I'm curious to see what the 'shells' look like smocked...