"Cockles," mentioned in several parts of the novel, are a kind of small clam. The familiar expression, "warms the cockles of his heart" (used by Bloom in Hades), apparently derives from the deep satisfaction of eating a good helping of these bivalves.
In Nausicaa Gerty MacDowell recalls eating "stewed cockles and lettuce with Lazenby's salad dressing for supper" with her father, and Bloom thinks of "cockles and periwinkles," the latter a kind of edible sea snail.
The people whom Stephen sees on the beach in Proteus
are all harvesting cockles, despite the dangerous sewage contamination in this
part of Dublin Bay. When he sees two older women come down
"the steps from Leahy's terrace" and wonders, "What has she in
the bag?," he fancies that they are midwives toting "A
misbirth with a trailing
navelcord." In fact, though, they have brought the bag with
them to hold their day's catch, as we learn in Wandering
Rocks: "Two old women fresh from their whiff of the
briny trudged through Irishtown along London bridge road, one
with a sanded tired umbrella, one with a midwife's
bag in which eleven cockles rolled."
Later in the chapter, Stephen recognizes a gypsy-looking couple as "Cocklepickers. They waded a little way in the water and, stooping, soused their bags and, lifting them again, waded out." Tossing their laden bags over their shoulders, they move on, and Stephen thinks of the "Spoils slung at her back."
Cockles dwell only an inch or so under the surface of the sand, so they are much easier to dig than larger clams.