SUB-CLASS INARTICULATA Valves without teeth or sockets but held together by muscles. Intestine long, ending in anus.
Order I. Atremata, Beecher emend. Schuchert.
Primitive forms, having horny or calcareous and phosphatic valves, with the pedicle emerging either between the two valves, or through a cleft usually common to both valves. Pedicle opening frequently closed by plates. Families—Rustellidae, Paterinidae, Schuchertinidae, Kutorginidae, Curticiidae, Obolidae, Trimerel lidae, *Lingulidae, Lingulasmatidae.
Order II. Neotremata, Beecher emend. Schuchert.
Shell usually horny, upper valve often conical. Pedicle emerg ing through an open cleft or through a perforation in the pedicle valve, closed by a special plate or listrium. Shell may be cemented to foreign objects. Families—Obolellidae, Siphonotretidae, Acro tretidae, Trematidae, *Discinidae, *Craniidae.