sentences:  Six genera have been identified.They possess a unique feeding mechanism among cetaceans known as suction feeding.They are characterized by having a lower jaw that extends at least to the tip of the upper jaw, a shallow or non-existent notch between the tail flukes, a dorsal fin set far backwards, three of four fused neck vertebrae, extensive skull asymmetry and two conspicuous throat grooves forming a 'V' pattern (which aid in sucking).