21. In the 18th century involute gears, another mathematical derived design, came into use. 22. The problem employs the " study of involute cycloidal, epicycloidal and hypocycloidal curves ." 23. The Nautilidae are involute or slightly evolute and generally smooth with straight to sinuous sutures. 24. It varies continuously along the involute profile. 25. "Zetoceras " has a compressed involute shell with a very small umbilicus. 26. Members of the Phylloceratidae are characterized by smooth, involute shells with very thin walls. 27. An iron-involuted brooch of the La T�ne II type has also been found. 28. The shell of " Falcitornoceras " is strongly involute , lacking an umbilicus. 29. Shells may be either evolute or involute . 30. The implementation of the involute tooth yielded a standard gear design that provides a constant speed ratio.
