Hemicrustacea

a class of ancient, extinct, primitive arthropods. They were common from the Cambrian period to the Permian period. Outwardly they resembled the Cladocera and Notostraca. A solid spinal shield and a rodlike or lamellate caudal spine (a telson) were characteristic of the Hemi-crustacea. There were approximately ten genera. They lived in water.