SPORADIC EVOLUTION OF DORSIVENTRALITY IN RAPHID DIATOMS, WITH SPECIALREFERENCE TO LYRELLA-AMPHOROIDES SP-NOV

Amphoroid symmetry and lesser degrees of lateral asymmetry have evolve d in several groups of raphid diatoms where the majority of taxa are s ymmetrical (naviculoid). An example is in Lyrella Karayeva. Lyrella am phoroides sp. nov. has a dorsiventral frustule but possesses valve, ra phe and chloroplast characteristics that show unambiguously that it be longs in Lyrella, where the majority of species are naviculoid. L. amp horoides is one of the commonest Lyrella species in marine sandy habit ats around British coasts. Sporadic evolution of amphoroid symmetry se ems to be associated primarily with sandy or rocky habitats. Reasons f or this and constraints on the evolution of dorsiventrality are discus sed.