 An alternative systematic scheme exists using gill morphology . This distinguishes between Protobranchia , Filibranchia and Eulamellibranchia . The first corresponds to Newell 's Palaeotaxodonta and Cryptodonta , the second to his Pteriomorphia , with the last corresponding to all other groups . In addition , Franc separated the Septibranchia from his eulamellibranchs because of the morphological differences between them . The septibranchs belong to the superfamily Poromyoidea and are carnivorous , having a muscular septum instead of filamentous gills . 
