Although the original description by H. Adams & A. Adams (1858) is brief, Ctenocardia was first recognized as distinct from other cardiids due to a heart-shaped form, an abruptly truncate anterior, a carinate beak and spinose ribs, this latter character being the main character differentiating the genus from other subfamily members. Much later, Habe (1951) described the genus Microfragum as having a small, flabelliform shell, with a more or less lobate posterior margin, not noticeably convex and without spikes on the radial ribs. This genus was erected in order to differentiate the type species, Cardium festivum Deshayes, 1855, from other members of the subfamily Fraginae. However, similarities among some members of Ctenocardia and Microfragum were evident and first recognized by Prashad (1932). Although Prashad (1932) did not include M. festivum in the subgenus Ctenocardia, the form and general facies of M. festivum were noted as similar to Cardium (Ctenocardia) fornicatum Sowerby, 1841. After examination of the type specimens of both genera by us, it is clear that these taxa share many shell features in common, yet are still distinct. The rib lamellae though similar to other members of Ctenocardia in being "mosaistracal" (sensu Vokes 1977) in nature, are distinct in being less spinose, thinner and more flattened and serried. Similarly, the somewhat quadrate and keeled gross shell shape, most pronounced in juvenile specimens of M. festivum, is reminiscent of other Ctenocardia members, but adult M. festivum are less keeled and more flabelliform (although see C translatum (Prashad, 1932)) than Ctenocardia. Finally, the presence of a second posterior lateral on the right valve is present in Microfragum, but less pronounced and less blade-like than in other Ctenocardia species, where it forms a pocket with the anterior lateral. To formally recognize both these similarities and differences, we propose relegating Microfragum to a subgenus of Ctenocardia, with C. (M) festivum and C. (M.) subfestivum n. sp. the only taxa presently allied to this subgenus. "Microfragum erugatum (Tate, 1889) is not a member of this group and belongs in Fragum, as recognized by Vidal (2001), but in contrast with Schneider (1998a). "Microfragum" ebaranum (Yokoyama, 1927) belongs in the genus Afrocardium, and is a synonym of A richardi (Audouin, 1827) (see Vidal 2000b).