78 ROTELLA. TROCHUS. TURBO. 



Rotella Trochus — wheel-shell. 



TrocJius Trochus — top shell. 



Monodonta. 



Turbo Turbo. 



Planaxis *. Buccinum. 



Phasianella Turbo. 



Turritella Turbo. 



" The genus Rotella contains small, flattened, 

 wheel-shaped shells, common in most collections. 

 They are smooth and polished. 



'' Trochus is still a large genus. Trochus mar- 

 mordtus is a fine species from the Indian Ocean. 

 There are several handsome species on our own 

 shores. T. magus is one of them ; it has a large, 

 deep umbilicus, or perforation ; the spire is flat- 

 tened ; the whorls are crowned with tubercles. 

 The Trochi of tropical climates are thinner than 

 those of northern latitudes. 



" When any of these shells are placed upon 

 their base.^ their axis is always inclined : of course 

 they never stand perfectly upright. 



" There are several fossil species. 



*' The genus Monodonta holds a middle place 

 between Trochus and Turbo, differing from the 

 former in the aperture, and from the latter in the 

 columella, which is arched and truncated at the 

 base. They are all marine shells. 



" From the well-known genus Turbo^ a new 



