. Bulletin of the Natural History Museum Zoology. DEEP-SEA PROTOBRANCHIA (BIVALVIA) 15. Fig. 4 Yoldiella lucida. Hinge plates of a, right valve of a specimen from Sta. 105 (North America Basin); b & c, right and left valve of specimens from Thor' Sta. 28 to show variation in the shape of the ligament. (Scales = mm). specimens the antero-dorsal margin is more convex, the postero-dorsal margin is normally angulate and the posterior margin more rounded (Fig. 4). As in many species of Yoldiella older specimens tend to become subrostrate. Prodissoconch length: 198 Maximum recorded shel
. Bulletin of the Natural History Museum Zoology. DEEP-SEA PROTOBRANCHIA (BIVALVIA) 15. Fig. 4 Yoldiella lucida. Hinge plates of a, right valve of a specimen from Sta. 105 (North America Basin); b & c, right and left valve of specimens from Thor' Sta. 28 to show variation in the shape of the ligament. (Scales = mm). specimens the antero-dorsal margin is more convex, the postero-dorsal margin is normally angulate and the posterior margin more rounded (Fig. 4). As in many species of Yoldiella older specimens tend to become subrostrate. Prodissoconch length: 198 Maximum recorded shell length: mm. Internal morphology. (Figs. 5 & 6) The mantle margin has a well-developed anterior sense organ. Posteriorly the siphons are combined, the inhalent siphon being open ven- trally. A well-developed feeding aperture lies immediately ventral to the siphons with numerous gland cells present in what is a broadened region of the inner muscular lobe. A long siphonal tentacle originates usually on the left side of the siphonal embayment near the base of the siphon. The adduc- tor muscles are large and unequal in size. The posterior muscle is oval in cross-section and between 1/2 and 1/3 the size of the anterior which is more circular in section. The visceral and cerebral ganglia are relatively large, club-shaped, with an exceptionally thick connecting commis- sure. The pedal ganglia are also large but more round in outline, and each with a large statocyst dorsal to it. The foot is anteriorly directed with well-developed pedal retractor muscles. The byssal gland is moderate in size. The gills are well-developed with between 16-23 gill filaments, the num- ber depending on the size of the individual. Posteriorly the gill axes attach to the junction between the two siphons. The labial palps are moderately large extending approximately a third across the body with between 13-23 palp ridges on the inner face. The palp proboscides are well-developed and are long and muscul
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