Most sharks have five gills.  Cow Sharks have either six or seven and are sometimes referred to as Sixgill (Hexanchus sp.) or Sevengill (Notorhynchus sp.) Sharks.  They live primarily in very deep water (a mile or more below the surface).  That's one of the reasons their teeth are so rare.

These fossil Cow Shark teeth are EXTREMELY fragile.  They have large, very-thin roots, which are often broken during excavation or cleaning.  You may be able to pick-up a repaired tooth at a reasonable price (we have a few).  Expect to spend a chunk of change though, to purchase a large near-perfect tooth.

Note - Notorhynchus is sometimes incorrectly spelled without the first "h" as Notorynchus.