The arthropod Alalcomenaeus cambricus Simonetta, from the Middle Cambrian Burgess Shale of British Columbia

More than 300 specimens of the previously rare arthropod Alalcomenaeus camb ricus Simonetta have been collected from a new Burgess Shale locality in th e Glossopleura Zone on Mount Stephen, British Columbia. This new material p rovides 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 thre e long flagella. The two posterior head appendages, like those of the trunk , were biramous. They consisted of a segmented, inner branch, and a flap-li ke outer branch, fringed with long filaments. The trunk consisted of 11 som ites, each protected by a tergite and bearing a parr of biramous limbs. The telson was paddle-like and fringed posteriorly with wide flat spines. Alal comenaeus was probably a predator, moving mainly by swimming. It is now kno wn to be one of the more abundant, widely distributed and longest ranging o f Burgess Shale arthropod genera. Its affinities lie with the Arachnomorpha .