monothalamia

from The Century Dictionary.

• noun A division of reticulate amœbiform protozoans, or Foraminifera, containing those whose test is single-chambered: opposed to Polythalamia. The term does not indicate any natural division of the foraminifers. See cut under Foraminifera.
• noun In conchology, a division of Cephalopoda, containing those cephalopods whose shell is single-chambered, as the genus Argonanta.