The Collaborative International Dictionary of English v.0.48:

slit-shell \slit"-shell`\ (sl[i^]t"sh[e^]l`), n. (Zool.)
   Any species of Pleurotomaria, a genus of beautiful, pearly,
   spiral gastropod shells having a deep slit in the outer lip.
   Many fossil species are known, and a few living ones are
   found in deep water in tropical seas.
   [1913 Webster]