Hysteroconcha multispinosa
Similar to Hysteroconcha dione. Hysteroconcha multispinosa has more widely spaced, sharper and more prominent comarginal ribs than H. dione. These two species are very similar morphologically, but they differ in 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 lupanaria. Hysteroconcha multispinosa is distinguishable by its prominent, erect comarginal ribs; whereas, H. lupanaria has less prominent ribs that are flattened in the central area of the flank, creating a smoother surface. H. multispinosa also has more spines than H. lupanaria.
The overall shape is subtrigonal, ovate. The posterior margin is subtruncated, and weakly pointed at the confluence of the posterior and ventral margins. 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 slightly more inflated than the anterior half. The valves are equal in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and rigid, prominent comarginal ribs. The ribs are sharp and thin; the space between the ribs is wider than one individual rib. At the anterior, every other rib is raised, forming prominent lamellae. At the posterior, every other rib forms a spine. There are two rows of spines at the posterior. The anterior row stretches the entire flank from dorsal to ventral margins; whereas, the posterior row is much shorter. The lunule and escutcheon are smoother than the rest of the flank. The lunule is lightly impressed, defined by a fine shallow groove and extends 1/3 of the anterior dorsal margin. The umbo is anterior, and the beaks also point slightly to the anterior. The escutcheon is slightly raised, defined by a fine shallow groove and extends 3/4 the posterior dorsal margin.
The exterior is white with purple and/or brown coloration comarginally, typically on the ventral side of the ribs. The ventral sides of the bases of the spines may also be purple. The spines are typically white or dirty gray. The lunule and escutcheon may also have purple and/or brown coloration. The interior is white with faint purple coloration along the posterior dorsal margin and under the nymph.
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 (2a, 2b, 4b), none of which are bifid. The anterior (2a) tooth is weak and the central tooth (2b) is robust; they are joined dorsally, forming an inverted "v" shape. The posterior tooth (4b) is fused with the nymph. 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) on the right valve, as well as a fine ridge running parallel to the posterior dorsal margin. The adductor muscle scars are similar in area, but the anterior is slightly more elongate. The pallial sinus is deep, rounded and extends to the middle of the nymph in length.
Hysteroconcha multispinosa is found in the Pacific Ocean, from Mexico to Tumbes in northern Peru (Olsson, 1961).