Type species. Cardita borealis Conrad, 1831 (pl. 8, fig. 1) (subsequent designation by Stolickza, 1871). Pleistocene–Recent, Northwest Atlantic Ocean.
Diagnosis. Carditid with thick shell, subcircular outline and convex dorsal margin. Umbo rounded. Right anterior tooth small, middle tooth very curved, broad and thick. Left anterior tooth large and posterior tooth strong and very curved. External sculpture of 15 to 27 entire and smooth radial ribs, that are low and covered by small subcircular or subrectangular nodes. Periostracum coarse and hirsute in living species.
Included species. Cyclocardia is an Eocene to Recent cosmopolitan genus that includes more than a hundred species (Maxwell 1969; Coan 1977; Stilwell & Zinsmeister 1992; Kafanov et al. 2001; Huber 2010; Güller & Zelaya 2013).
Remarks. Conrad (1867) highlighted that this genus was characterized by a thick shell with a coarse periostracum, generally smooth radial ribs and strongly inclined posteriorly hinge teeth. Subsequently, Stewart (1930) indicated that Cyclocardia has grooved hinge teeth, but that character is not found in all species.
According to Heaslip (1968), Cyclocardia has some convergent characters with planicostate carditids, such as the development of a thick shell with low and smooth radial ribs, and pointed out that the sculpture of juvenile specimens is similar to alticostate carditids, inferring that Cyclocardia could have originated within this group.
The most ancient records of Cyclocardia are represented by the Eocene C. mesembria Stilwell & Zinsmeister, 1992 (Marambio Island, La Meseta Formation, Antarctica), C. steinekei Clark & Dunham, 1946 (El Carmen District, unnamed formation, Colombia), C. otatsumei Uozomi, 1955 (Hokkaido, Wakkanabe Formation, Japan), C. ezoensis (Takeda, 1953) and C. hamiltonensis (Clark, 1932) (Kamchatka, Aluginskaya Formation, Russia) and by C. caumontiensis (Cossmann & Pisarro, 1904) (Paris Basin, France).
Cyclocardia is represented by four Patagonian species (C. cannada, C. dalek n. sp., C. nortensis and C. compressa) and is the most diverse carditid genus in the region. According to the large amount of intraspecific variability on this genus, Huber (2010) proposed the existence of at least three subgenera within Cyclocardia. As already discussed by Maxwell (1969), New Zealand representatives may belong to a different lineage or genus (see ‘ C. ’ nortensis below).