A scute or scutum (Latin: scutum; plural: scuta "shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterior portion of the mesonotum in insects as well as some arachnids (e.g., the family Ixodidae, the scale ticks). Properties Scutes are similar to scales and serve the same function. Unlike the scales of lizards and snakes, which are formed from the epidermis, scutes are formed in the lower vascular layer of the skin and the epidermal element is only the top surface . Forming in the living dermis, the scutes produce a horny outer layer that is superficially similar to that of scales. Scutes will usually not overlap as snake scales (but see the pangolin). The outer keratin layer is shed piecemeal, and not in one continuous layer of skin as seen in snakes or lizards. The dermal base may contain bone and produce dermal armour. Scutes with a bony base are properly called osteoderms. Dermal scutes are also found in the feet of birds and tails of some mammals, and are believed to be the primitive form of dermal armour in reptiles. The term is also used to describe the heavy armour of the armadillo and the extinct Glyptodon, and is occasionally used as an alternative to scales in describing snakes or certain fishes, such as sturgeons, shad, herring, and menhaden. Mammals Prehistoric ancestors of mammals, the synapsids, are thought to have scutes which were later reduced and replaced by hair. Excluding the attachment surface of fingernails, armoured scutes or scales are almost never seen in modern mammals. The horny scales of pangolins are only rarely called scutes, but "scute" is used to describe the heavy armour of the armadillo. Turtles The turtle's shell is covered by scutes formed mostly of keratin. They are built similarly to horn, beak, or nail in other species. Fish Some fish, such as pineconefish, are completely or partially covered in scutes. River herrings and threadfins have an abdominal row of scutes, which are scales with raised, sharp points that are used for protection. Some jacks have a row of scutes following the lateral line on either side. Sturgeon have five rows of scutes instead of scales. Birds The tarsometatarsus and toes of most birds are covered in two types