The ground sharks, order Carcharhiniformes, are the largest order of sharks. With over 270 species, carcharhiniforms include a number of common types, such as catsharks, swellsharks, and the sandbar shark.
False cat sharks are a small family containing false catsharks and gollumsharks. It contains the only ground shark species that exhibit intrauterine oophagy, in which developing fetuses are nourished by eggs produced by their mother.
Catsharks are distinguished by elongated cat-like eyes and two small dorsal fins set far back. They usually have a patterned appearance, ranging from stripes to patches to spots. Most are fairly small, growing no longer than 80 cm (31 in); a few, such as the nursehound can reach 1.6 m (5.2 ft) in length. They are found in temperate and tropical seas worldwide, ranging from shallow intertidal waters to depths of 2,000 metres (6,600 ft) or more, depending on species. They feed on invertebrates and smaller fish. Some species are aplacental viviparous, but most lay eggs in tough egg-cases with curly tendrils at each end, known as mermaid's purses. The swell sharks of the genus Cephaloscyllium fill their stomachs with water or air when threatened, increasing their girth by a factor of 2 or 3. Some catsharks are called dogfish.
Hammerhead sharks are named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Many not necessarily mutually exclusive functions have been proposed for the cephalofoil, including sensory reception, manoeuvring, and prey manipulation. Hammerheads are found worldwide in warmer waters along coastlines and continental shelves. Unlike most sharks, hammerheads usually swim in schools during the day, becoming solitary hunters at night.
Houndsharks are distinguished by large spineless dorsal fins, an anal fin, and oval eyes with nictitating eyelids. They are small to medium in size, ranging from 37 centimetres (15 in) to 220 centimetres (7.2 ft) in adult length. They are found throughout the world in warm and temperate waters, where they feed on fish and invertebrates on the seabed and in midwater.