An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the eggs of oviparous sharks, skates and chimaeras.

Egg cases typically contain one embryo, except for big skate and mottled skate egg cases, which contain up to 7 embryos.

Oviparity in sharks can be categorized as single or retained. With single oviparity, the egg cases are extruded soon after fertilization. With retained oviparity, eggs are kept within the oviduct for a period of time before depositing outside of the body as an unhatched egg case.

It is thought that viviparity is the ancestral condition for sharks, and that it evolved through the elongation of retention time of retained oviparity.

Oviparous sharks are known to regularly produce unfertilized eggs when kept in captivity without males.