A specific example of a transition I am familiar with: the Yorktown Formation ~4.5-3.2 MYA Melanella laevigata, based on the photograph in Campbell, 1993, has 11-13 teleoconch whorls when the shell is 10 mm long, and has a very weak suture. The similar recent species M. jamaicensis has 8-9 teleoconch whorls at that size and has a somewhat stronger suture. Three specimens that I have from the 2.4 MYA Waccamaw Formation have 9-11 whorls at that size and an intermediate-strength (still weak) suture.