Traditionally, trilobites are subdivided into two major groups: the polymeroids and the agnostoids. Polymeroids are garden-variety trilobites. They have a multisegmented thorax; they usually possess eyes; and they can be macropygous, isopygous, or micropygous (the head is smaller than the tail, the head & tail are equal-sized, or the head is larger than the tail). Agnostoids have a two-segmented thorax only; they are all blind; and they are all isopygous (the heads & tails are equal-sized). Agnostoids are usually quite small.
Many modern arthropod phylogenetic studies conclude that agnostoids should be excluded from the trilobites.
Below are two specimens of the common agnostoid Peronopsis interstricta (White, 1874). They come from the upper Middle Cambrian Wheeler Formation of the House Range in western Utah.
Classification: Arthropoda, Trilobita, Agnostida, Peronopsidae
Peronopsis interstricta (8.5 mm long each) (commercially prepared) from the Wheeler Formation (Middle Cambrian) of western Utah’s House Range.