Agnostida.

Agnostid trilobites, or Agnostida, are bizarre trilobites that lived from the early Cambrian to the Ordovician mass extinction. In some Cambrian rocks, Agnostids are extremely common, and they are also spread all over the world in the fossil record. But in the Ordovician period, Agnostids become rare. But I believe they were probably still widespread.

A few Agnostid trilobite drawings of different species. [Artist unknown].

Most, but not all, Agnostids were blind, and they all were isopygous (meaning the pygidium, or tail, was the same size as cephalon, or head). Unlike other trilobites, Agnostids only had two or three thoracic segments, and like the order Nekaspida, which include Naraoia, some scientists do not believe that they are even trilobites. Instead they think that both these orders are more closely related to crustaceans than trilobites, but not stem group or crown group crustaceans.

Agnostid legs were unique among the appendages of trilobites, because most trilobites have legs similar to those of a pill bug. Agnostids have their own unique arrangement of legs and other appendages.

The ventral view of Agnostus, showing the legs and other appendages. 

Scientists disagree about whether Agnostids were benthic or pelagic. They seem to be benthic, because they are found with other benthic trilobites, and also have no eyes, which suggests that they would not be living at the surface where there is a lot of sunlight to help them see. But some scientists think they were pelagic because they are widespread, and usually pelagic animals can get to places more quickly and easily than benthic animals.

Some Agnostids could have been benthic predators that cannibalized and hunted in packs. Agnostids can commonly be found sheltered inside the remains of dead animals, including the abandoned tubes of the priapulid Selkirkia. Since the ones found in Selkirkia all have their head facing out of the tube, it indicates that they must have backed in for shelter and protection. Since the molted skins of Agnostids have not been found inside the tubes of Selkirkia, it means that they probably did not go into the tubes to molt.

References:

http://www.ucmp.berkeley.edu/arthropoda/trilobita/agnostida.html

http://www.trilobites.info/ordagnostida.htm

http://en.wikipedia.org/wiki/Agnostida