In 1843 Simpson published a description of a Yorkshire Ammonite previously named Ammonites trivialis in Bean's manuscript. A few years later Quenstedt described a number of small continental Ammonites of similar character as A. polymorphus, which he further divided into the varieties lineatus, costatus, interruptus, mixtus, and quadratus. The specimens he then figured must be taken as his types; in a later work he showed many more examples which he referred to the same species; all these have small, evolute shells, but few other common characters, so that the name polymorphus was fully justified. For example, the illustrations given as Ammonites polymorphus mixtus include forms with round or square whorls, with nearly straight or with curved ribs, which may pass across the venter with or without a forward bend, or may be suppressed entirely on the venter. The specimens figured probably include young examples of several different genera.