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Taxonomy
Cardium subalternatum was named by Vogel (1895). It is not extant.
Sister species lacking formal opinion data
C. alabamense, C. cedrelarum, C. cottaldinum, C. erinaceum, C. heinzelini, C. hernandoense, C. hispidiforme, C. knappi, C. latisulcum, C. leucostoma, C. lineolatum, C. lithopodolicum, C. problematicum, C. rouyanum, C. scabrum, C. schweinfurthi, C. transcarpaticum, C. triangulatum, C. vindobonense, C. volckmanni, C. colosseum, C. estacianum, C. sellardsi, C. triboleti, C. hanpeizanense, C. anclotense, C. brooksvillense, C. coutinhoi, C. hernandoense, C. hortensium, C. leucostomum, C. plectopleura, C. ustrix, C. viridarii, C. colosseum