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More than 300 specimens of the previously rare arthropod Alalcomenaeus cambricus Simonetta have been collected from a new Burgess Shale locality in the Glossopleura Zone on Mount Stephen, British Columbia. This new material provides much more complete information on its morphology. The cephalon was covered by a shield. A pair of pedunculate eyes and three median eyes were followed by a large anterior appendage, the ‘great appendage’, bearing three long flagella. The two posterior head appendages, like those of the trunk, were biramous. They consisted of a segmented, inner branch, and a flap-like outer branch, fringed with long filaments. The trunk consisted of 11 somites, each protected by a tergite and bearing a pair of biramous limbs. The telson was paddle-like and fringed posteriorly with wide flat spines. Alalcomenaeus was probably a predator, moving mainly by swimming. It is now known to be one of the more abundant, widely distributed and longest ranging of Burgess Shale arthropod genera. Its affinities lie with the Arachnomorpha.