Acteocina canaliculata (Say, 1826)
Acteocina atrata P.S. Mikkelsen and P.M. Mikkelsen, 1984
Acteocina candei (d'Orbigny, 1841)

Most collectors (including myself) have difficulty differentiating these species.  Many collectors are still unaware of Acteocina atrata named only in 1984.  A. candei is not often found near shore and I've never found it in a lagoonal area.  A. canaliculata and A. atrata are found in the same habitat (esp. lagoonal areas) and often together.  And, they are essentially indistinguishable except for the protoconchs. If you find an Acteocina in a Florida lagoon, chances are extremely high you are dealing with A. canaliculata or A. atrata. For more information on these species see: 

Comparison of Acteocina canaliculata (Say, 1826), A. candei (d’Orbigny, 1841), and A. atrata spec. nov. (Gastropoda: Cephalaspidea), Paul S. Mikkelsen and Paula M. Mikkelsen, The Veliger, 27(2): 164-192, October 5, 1984.

I regret that I have no presentable images of A. atrata. However, below are presentations of A. candei and A. canaliculata courtesy of José H. Leal, Science Director & Curator, Bailey-Matthews National Shell Museum .