Believed extinct for about 65 million years but were discovered in 1855. To date (January 2018) only 35 species are known, but some are still uncertain. As far as we know, no museum in the world has them all, because some of them know only a few specimens. In the collections of the Malakos Museum there are as many as 23 species with over 60 specimens.
Today we know more than 1000 fossil species, but in the light of new knowledge, it is very necessary to have an accurate systematic review to eliminate the numerous synonymies.
Many times the bad state of fossil conservation makes some morphological characters, necessary for a correct identification of the species, invisible. The error of interpretation is always lurking.