pleurotoma

from The Century Dictionary.

• noun In conchology, the typical genus of Pleurotomidæ: so called from the notch or slit in the outer lip of the aperture.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) Any marine gastropod belonging to Pleurotoma, and ether allied genera of the family Pleurotmidæ. The species are very numerous, especially in tropical seas. The outer lip has usually a posterior notch or slit.