thumb|upright=1.2|right|Egg case of a [[Skate (fish)|skate]]
thumb|Embryo active inside egg case
An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the eggs of oviparous chondrichthyans. Living chondricthyans that produce egg cases include some sharks, skates and chimaeras. Egg cases typically contain one embryo, except for big skate and mottled skate egg cases, which contain up to seven embryos. and are often described as feeling rough and leathery. Egg cases without a fibrous outer layer can be striated, bumpy, or smooth and glossy. With single oviparity, the egg cases are extruded soon after fertilization.
Oviparous sharks are known to regularly produce unfertilized eggs when kept in captivity without males.
Bullhead sharks
Bullhead shark egg cases are shaped like an auger, with two spiral flanges. This allows the egg cases to become wedged in the crevices of rocky sea floors, where the eggs are protected from predators; however, some bullhead sharks deposit their eggs on sponges or seaweed. Hatchlings are considered large for sharks, reaching over 14 cm in length by the time they leave the egg case. Females lay egg cases onto the sea floor after fertilization occurs in utero. While in utero, a protected case forms around the embryo which is called the egg case. Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species.
Longnose skate
The longnose skate, Raja rhina, is considered a larger skate species; reaching a maximum size range of 145 cm total length. The egg cases of chimaeras are spindle- or bottle-shaped with fins on the sides. They are laid on the bottom of the sea floor. Chimaeras (subclass Holocephali), some sharks, and skates are among the 43% of known Chondrichthian species to exhibit oviparity. However, there are some key morphological differences that are specific to chimaeras. The holocephalan egg capsule, or egg case, has a bulbous center flanked laterally by flattened collagen tissue. The flattened collagen tissue joins on the anterior end of the egg capsule to form a tail.
Extinct chondrichthyans
thumb|Illustration of an Early Jurassic estuarine ecosystem, with [[Palaeoxyris egg cases attached via tendril to Neocalamites stems, with hybodonts in the background]]
The egg case genera Palaeoxyris and Fayolia, which are thought to have been produced by hybodonts and xenacanths respectively, two groups of extinct shark-like cartilaginous fish more closely related to modern sharks and rays than to chimaeras, resemble those of bulldog sharks in having a spiral collarettes running around them. Both Palaeoxyris and Fayolia taper towards their ends (with the tapering being more pronounced in Palaeoxyris), with one end having a tendril. Unlike modern sharks, these eggs are typically found in freshwater environments.
Threats
Predation on egg cases is thought to be a major source of mortality for developing oviparous sharks, skates and chimaeras. In general, predation is the leading cause of mortality for marine fish eggs, due to their abundance and high nutritional value.