Hysteroconcha dione
Similar to Hysteroconcha lupanaria. Hysteroconcha dione is distinguishable by its more erect, thinner comarginal ribs that rarely split; H. lupanaria has wider ribs that split posteriorly and are flattened across the central area of the flank, resulting in a smoother surface. H. dione has more spines than H. lupanaria, but they are more likely to be broken off. In addition, their distributions are different: H. dione is found in the Carribean, while H. lupanaria is found in the Pacific (Olsson 1961).
Similar to Hysteroconcha multispinosa. Hysterconcha dione typically has comarginal ribs that are slightly closer together and less rigid than those of H. multispinosa. Morphologically, these two species are very similar, with the main difference being their geographic distribution: H. dione is found in the Carribean and Atlantic Ocean, and H. multispinosa is found in the Pacific (Olsson 1961).
Similar to Hysteroconcha rosea. Hysteroconcha dione is distinguishable by its sharper ribs, wider area between its ribs and greater number of anterior lamellar projections. H. dione also has two rows of posterior spines; whereas, H. rosea only has one.
The overall shape is subtrigonal ovate. The posterior margin is subtruncated, and the anterior and ventral margins are rounded. The anterior dorsal margin is subconcave, and the posterior dorsal margin is subconvex. The posterior half of the flank is more inflated than the anterior half. The valves are equal in shape and size (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and prominent, erect comarginal ridges; the spaces between the ridges are wider than the individual ridges. At the anterior, the ridges become more prominent, and some (typically every other ridge) form lamellae. At the posterior, the ridges form two rows of erect spines that are often broken off. The anterior row extends to the shell margin and has prominent spines forming from every third or fourth ridge. The posterior row typically does not extend to the shell margin, and its spines are weaker than those of the anterior row. The lunule, peri-lunule and escutcheon lack ridges and are smoother than the rest of the flank. The lunule is lightly impressed, defined by a fine shallow groove and extends 1/5 of the anterior dorsal margin. The umbo is anterior and the beaks also point slightly to the anterior. The escutcheon is smoother than the central area of the flank, slightly raised and defined by a fine, shallow groove.
The exterior is white with purple coloration along the comarginal ridges, and at the anterior and posterior sides of the bases of the spines. The lunule, peri-lunule and posterior area of the flank, including the escutcheon, are covered by purple or brown coloration. The spines are typically white or light brown and/or purple in coloration. The interior is white with purple coloration on the hinge plate, under the lunule and along the posterior dorsal margin.
The ligament is external, extends half the posterior dorsal margin and is supported by nymphal ridges. On the left valve, there are three cardinal teeth. The anterior tooth (2a) is weak and not bifid, but may be lightly grooved. The central tooth (2b) is robust and may be not bifid or lightly bifid. The anterior (2a) and central (2b) teeth meet dorsally, forming an inverted "v" shape. The posterior tooth (4b) is not bifid and fused with the nymphal ridge. There is also one anterior lateral tooth (A2) on the left valve. On the right valve, there are three cardinal teeth: the anterior (3a) and central (1) teeth are not bifid, and the posterior tooth (3b) is bifid. There are also two anterior lateral teeth (A1, A3) and a fine ridge running parallel to the posterior dorsal margin on the right valve. The adductor muscle scars are similar in area, but the posterior is rounder than the anterior. The pallial sinus is deep, slightly tapered, and extends to the middle of the nymph in length.