Amicus Koroleva, 1967 is mentioned, but PBDB assigns it to the Ptychopariida--->
The subclass Librostoma was created in 1990 by Richard Fortey to cover the orders formally included with the Ptychopariida.
These typically small trilobites resemble those of the order Ptychopariida, from which the new order Proetida was only recently separated in 1975 by Fortey and Owens.