There are anterior, posterior, and two lateral thickenings. Taenidia generally take the form of either hoop or spiral thickenings of the tracheal cuticle. Before it has split, four thickenings occur. They are complete cells, usually with helical thickenings at maturity that respond to moisture content. The olfactory placode forms as two thickenings of non-neural region of embryonic ectoderm. Some, also, have keels, reinforcing thickenings along exterior angles. The elater cells do not grow helical thickenings. Local thickenings (often called warts) are common, such as those found on toads. The ribs sometimes develop thickenings or spines and give the shell a rough appearance. Also known as "papillae", they are downward thickenings of the epidermis between the dermal papillae. They are complete cells, usually without helical thickenings (except in the Dendrocerotaceae). The pseudopollen is produced from connective cells, and has either rib-like or circular thickenings. The thickenings are the origins of the three cusps of the semilunar valves. Epidermal thickenings called Rete ridges (or rete pegs) extend downward between dermal papillae.