Craniida

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Treatise on Invertebrate Paleontology, Part H, Vol. 2 (2000):

• Dorsal valve invariably calcified, punctuate; ventral valve in some taxa without secondary shell or
  entirely periostracal; mantle setae present in recent forms; shell growth generally holoperipheral, rarely
  mixoperipheral; dorsal valve convex to subconical, ventral valve plane to convex or encrusting; dorsal
  body wall of recent forms attached to dorsal valve in five separate areas; attachment scars of dorsal
  internal oblique muscles situated posterolaterally; mantle canal system pinnate, only rarely with vascula
  media

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