"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.

JH 2015

Cockle shells on the beach. Source:

Periwinkle emerging from its shell in Sweden, photographed in 2005 by Guttorm Flatabø. Source: Wikimedia Commons.