Shell small (H = 10-20 mm), inequilateral, with posterior slope straight to slightly concave, markedly depressed and separated from the median area by a strong keel; medio-posterior slope rather variable rounded to moderately pointed, anterior margin rounded.
Lunular area narrow, with raised margin; lunule present but extremely small, situated just behind the anterior cardinal in the right valve.
Exterior grey-white to beige with irregularly dis-posed, concentrically arranged maroon-brown spots; on the posterior slope spots larger and more regularly disposed. Interior always with several maroon coloured zones, with a ray along the medio-posterior angle, a spot on the part of the lunular area close to the small lunule and a ray along the ventral margin of the hinge below the ligament.
Anterior laterals slightly closer to cardinals than to posterior laterals. Second posterior lateral tooth in the right valve rarely observed. Ribs 39-45; 10-12 on the posterior slope. Posterior ribs slightly trapezoid to squaresided, tops flatly rounded and much wider than the finely-striated interstices. Close, thin, arched threads on tops of ribs, at irregular intervals along the rib, some threads becoming broader, higher and longer, with pointed extremities overhanging interstices. These broader transverse nodes are irregularly distributed, touching to slightly separated by a variable number (up to 10) of thin threads. Anterior rib ornaments similar to posterior ones.

Ctenocardia gustavi n. sp. resembles the American Ctenocardia (Americardia) media (Linnaeus, 1758) and the American fossil species Trigonocardia decidua (Vokes, 1977) from the Oligo-Miocene Chipola Formation of Florida in several characters, including shape, hinge and rib morphology. In the tropical west Pacific, C. gustavi n. sp. is found sympatrically with six recognized conspecifics: C.fornicata (Sowerby, 1841), C. victor (Angas, 1872), C. translatum, C symbolica Iredale, 1929, C perornata Iredale, 1929 and C. fijianum n. sp. Although these taxa are similar in gross shell shape, rib number and distribution, they differ significantly in aspects of rib morphology.

Widespread in the tropical West Pacific from Mariana and Caroline Islands to Indonesia (Sulawesi), Papua New Guinea, New Caledonia and Niue Island. Ctenocardia gustavi n. sp. has been recovered from depths of 1 to 54 m. It has been collected live by one of us (LK) in Papua New Guinea at a depth of 40 m, where it was shallowly buried on a narrow coral rubble rock ledge.