oped, if at all, and there is no ventral ridge, the flattened 

 bottom of the carapace being either gently convex or slightly 

 concave. At the opposite extreme to the forms with a 

 triangular carapace is Ostracion, with a rectangular 

 carapace in which the distance between the dorso- 

 lateral ridges is about equal to that between the ventro- 

 lateral ridges, and with both the back and belly flattened 

 or only gently convex, the dorsal and ventral ridges 



