However, there are a number of fundamental problems with this theory. Although the secreted shell of the female argonaut certainly bears a superficial resemblance to the ammonite, the argonaut shell differs not only in form but also in function and mineral composition. Their shells are mostly composed of calcite whereas the ammonite was mainly made of aragonite (mother of pearl). Additionally argonaut egg cases lack internal chambers and a siphuncle and are only created by the female, unlike those of the ammonite.