skates produce their young in an egg case called a mermaid's purse. these egg cases have distinct characteristics that are individualized to each species. this makes a great tool for identifying different species of skates. one of these identifiable structures, is the keel. the keel is a flexible ridge that runs along the outside of the structure. another characteristic is the number of embryos in the egg case. some species contain only one embryo while others can have up to seven. the size of the fibrous shell around the case is another characteristic. some species have thick layers on the exterior, however other species do not have the presence of this layer.