 <unk> have a prismatic and nacreous shell structure , separated mantle lobes , poorly developed siphons , and hinge teeth that are lacking or unspecialized . Gills range from <unk> to <unk> . <unk> on the other hand have a <unk> and partly nacreous shell structure ; Mantle lobes that are generally connected , well developed siphons , and specialized hinge teeth . In most , gills are <unk> . 
