Cyclocardia compressa

Cyclocardia compressa (Reeve, 1843)

Figure 3.4–6

*1843 Cardita compressa Reeve, pl. 9, fig. 46.

1850 Cardita (Actinobolus) procera Gould, p. 416.3.

v 1907 Venericardia procera (Gould) —Ihering, p. 425. 1933 Venericardia procera (Gould) —Feruglio, p. 107, pl. 9, figs. 5–8. 1964 Cyclocardia compressa (Reeve) —Dell, p. 190.

2000 Cyclocardia compressa (Reeve) —Pastorino, p. 156

2005 Venericardia procera (Gould) —Aguirre et al., p. 25, table B1

2013 Cyclocardia compressa (Reeve) —Güller & Zelaya, p. 203–205, figs. 1a–m, 9a.

Type specimens. Syntypes BNHM 1967582, three articulated shells from Valparaíso (Chile) and ‘Río Negro’ (Argentina).

Other material. MACN-Pi 363–364 (12 left and 15 right valves).

Diagnosis. Small-sized shell (less than 30 mm in length) laterally compressed. Radial ribs (15 to 17) wide, intercostal spaces half as wide as radial ribs (modified from Reeve 1843: pl. 9).

Description. Shell small with subcircular outline, laterally compressed; anterior and ventral margins rounded, posterior margin slightly convex and straight and dorsal margin long. Umbo placed near to middle of the valve length. Lunule small, deep, flat, longer than wide and bounded from remaining surface of shell by a groove.

Right valve hinge with very concave ventral edge; anterior tooth very small, slightly inclined posteriorly; middle tooth triangular, with broad base, very inclined posteriorly, concave anterior and almost convex posterior sides; posterior tooth slightly curved, as long as middle tooth. Left valve hinge with straight ventral edge; anterior tooth large, triangular, wide, straight, slightly inclined forwards with flat anterior and posterior sides; posterior tooth elongate, wide, high and inclined posteriorly.

External sculpture of 16 to 17 very low and entire smooth radial ribs with subelliptic transverse section, becoming obsolete towards ventral margin; shallow intercostal spaces up to half-width of radial ribs with subtriangular transverse section; last 5 to 6 posterior radial ribs narrower than the remainder, delimiting a very faint posterior zone. Pallial line one-fifth of total valve height. Inner ventral margin strongly crenulated; crenulations subrectangular, truncated, covering entire margin to ventral edge of posterior adductor muscle scar.

Remarks. Cyclocardia compressa was described by Reeve (1843) from Valparíso (Chile) and Cardita (Actinobolus) procera by Gould (1850) from ‘ Río Negro, Patagonia’; both species were synonymized by Dell (1964). Ihering (1907) mentioned its presence in ‘Pampean’ horizons of Puerto San Julián and Deseado River. Other Pleistocene mentions in the region were Aguirre et al. (2005) for Bustamante Bay (Chubut Province) and Pastorino (2000) for an unspecified locality of Río Negro Province.

Aguirre et al. (2005) proposed that C. compressa had migrated from Patagonia to Northern waters during the late Pleistocene or Holocene, but there are not Recent records of this species in Brazil (Ríos 1994) or along the coast of Buenos Aires Province (Castellanos 1967).

Cyclocardia compressa differs from C. borealis by having a smaller and more compressed shell, less numerous radial ribs and narrower intercostal spaces.