Cetoconcha Dall 1886

Cetoconcha Dall, 1886 Type species: Lyonsia bulla Dall, 1881, by original designation. Desciption Shell: Globular to ovate. Inequivalve; right valve overlaps less anterodorsally and posterodorsally. Inequilateral, posterior region with siphonal gape. Surface granulated, covered by numerous radial lines. Hinge of less valve edentulous and right sometimes with weak cardinal tooth corresponding to a socket in less valve. Inner ligament supported by elongated and superficial resilifer (over hinge margin), delimited by angular ridges. Lithodesma absent. Posterior hinge margin covered by a layer of fused periostracum. Remarks The following description of the soss parts of Cetoconcha is based on the information taken from the studied specimens of Cetoconcha spinulosa, and the bibliography (Pelseneer 1888 b, Allen and Morgan 1981, Krylova 2001, Leal 2008). The body is completely sealed, except for two siphonal and one pedal gape, similar to Cardiomya. However, the pedal gape in Cetoconcha is larger, running from the anterior adductor muscle to two-thirds of the ventral margin. Internally, the palial cavity is compartmentalized into an infraseptal and a supraseptal cavity by the muscular septum and connected through three pairs of groups of septal pores. The number of septal pores varies subtly, sometimes even between sides of the same specimen, and each one contains gill filaments lacking interfilamental connections. The cavities connect to the exterior through the siphonal gaps, as described in Cardiomya. In Cetoconcha, both siphons are exposed, instead of being inside a siphonal sheath, and they are separated by the intersiphonal septum (Fig. 5B). The inhalant siphon is modified into a large raptorial sac invaginated inside the pallial cavity, and the exhalant siphon is short, conical, and slightly prominent. In the scanned specimen, the raptorial inhalant siphon was fixed, contracted, and semi-evaginated near the siphonal tentacles, hence its shape cannot be understood fully (Fig. 5A–C). The siphonal tentacles are tapered and surround the siphons with an irregular distribution, with one dorsal to the exhalant siphon and 12–14 lateral and ventral to the inhalant siphon. Machado et al. (2019) reported 13 siphonal tentacles, but this number tends to vary among Poromyoidea, even within a species, commonly ranging between 13 and 15 (Krylova 2001). This numerical variation is evident among the studied specimens of Cetoconcha spinulosa. A pair of structures that seem to correspond to the pair of internal tentacles described by Allen and Morgan (1981) for Poromya and Cetoconcha species is located ventral to the posterior adductor muscle, anterior to the exhalant siphon and lateral to the visceral ganglion (Fig. 5B). Although Machado et al. (2019) did not mention any internal siphonal tentacles, they were observed in ZUEC 7002. Allen and Morgan (1981) proposed the cleaning of the supraseptal cavity from faecal material as a possible function. The septum is connected anteriorly to the mantle margin between the anterior adductor muscle and the anterior labial palps and, posteriorly, to the intersiphonal septum. One pair each of fine anterior, posterior, and lateral muscles provide attachment to the shell. The anterior and posterior septal muscles in Cetoconcha are much thinner and more rod shaped compared with Cardiomya. The lateral septal muscles are concentrated on the posterior side of the body, attaching to the shell at a single point. Posterior to the septal muscles, the septum becomes more membranous. At this point, a swollen and hollow bilobate sac, apparently associated with the eversion of the inhalant siphon (Morton 1981), is visible. Foot hatchet-shaped, large, with a shallow byssal groove, attached to the shell through one pair of anterior retractor muscles and one posterior muscle that bifurcates near the attachment site. The anterior palps are well developed, and the posterior palps are reduced in comparison but still noticeable (Fig. 5A, B, D). The digestive tube remains the same as in Cardiomya. The stomach is covered anterodorsally by the digestive diverticula, posterodorsally by the ovaries, and posteroventrally by the testes (Fig. 5A, B), because this genus is hermaphroditic. Only the visceral ganglia could be observed (Fig. 5E, F). ‘Visceral muscles’ are also present and can be seen in Figure 5A, B, F in our scanned specimen (MACN-In 44329) and in Figure 5G in the specimen from the study by Machado et al. (2019) (ZUEC 7002). These muscles are visible to the naked eye (Fig. 7F), and instead of forming a group of scarce muscle bundles, as in Cardiomya, they form a rod-shaped structure.