Cyclida

Cyclida (formerly Cycloidea, and so sometimes known as cycloids) is an extinct order of crab-like fossil arthropods that lived from the Carboniferous to the Jurassic and possibly Cretaceous. Their classification is uncertain, but they are generally interpreted as crustaceans, likely belonging to the superclass Multicrustacea.

==Description== left|thumb|Life restoration of Brittaniclus rankini Cyclidans have a "striking" resemblance to crabs, with circular to ovoid carapaces, which vary from flat to domed. The carapace covers the entire body. The largest members are over across the carapace. Their gill plates are arranged in a horseshoe shape beneath the carapace in at least some species. The head and thorax were fused into a single unit, the cephalothorax. The eyes, when present, were stalked, with the head also bearing two pairs of antennae, the first of which (dubbed the antennulae), are significantly longer than the second. There are at least eight pairs of appendages excluding the antennae and antennulae. The anterior appendages (including the pairs of maxillae and maxillipeds) are modified into chelae-like structures, with the posterior appendages being walking legs, which have sharp terminal segments (dactyli). The abdomen is short with either one or two segments, which ends with a pair of posterior structures, dubbed "caudal rami".