Shark eggs, also called "mermaid's purses," are shaped like long, tapering rectangles. They safeguard the growing embryo inside by becoming strong and leathery. Dark brown to black is the color range, and some species may have lighter patterns or markings.  The length can range from a few centimeters to over 20 centimeters, depending on the kind of shark. The majority of shark eggs share these common qualities, ensuring the survival of the developing embryo until it is ready to hatch even though each species may have particular traits.