Less frequently, the concave underside of the eggshell fragment will preserve bumps known as mammillae. It is composed of cone-shaped structures called mammillae at the base of each shell unit. The inner layer, called the mammillary layer, is made of tightly-packed cones called mammillae. The mammillae are proteoglycan-rich and are thought to control calcification. In the shell gland (similar to a mammalian uterus), mineralization starts at the mammillae. Mammillae are the first part of the eggshell to form.