'Rugalucina' cypselis (Melvill, 1918)

Taylor, John D. & Glover, Emily A., 2019, Unloved, paraphyletic or misplaced: new genera and species of small to minute lucinid bivalves and their relationships (Bivalvia, Lucinidae), ZooKeys 899, pp. 109-140 : 109

'Rugalucina' cypselis (Melvill, 1918) Figs 3 , 6

Divaricella cypselis Melvill, 1918: 156, pl. 5, fig. 33.

Type material.

D. cypselis holotype NHMUK 1921.1.28.42. sh, L 5.2 mm.

Type locality.

Karachi, Pakistan, 20-30 fathoms (36-55 m).

Other material examined.

Pakistan: Karachi, Winckworth collection (NHMUK 20191075) 55 sh, 52 v. India: dredged Chennai (Madras), Winckworth Collection (NHMUK 1958.1.30.45) 1 sh. Chennai (Madras) (NHMUK 1953.1.30.169-73 part) 5 v.

Description.

Small (L to 5.2 mm), sub-circular, inflated. Colour white or yellowish. Sculpture of diverging, curved, radial ribs prominent to anterior and posterior but are subdued or absent in middle parts of shell. Ribs crossed by closely spaced, narrow, low, commarginal lamellae that are aligned obliquely to the ventral shell margin ( Fig. 6M ). Radial ribs with small scales where crossed by commarginal lamellae. Early parts of shell relatively smooth. Protoconch P1 ca 88 µm, P2 ca 170 µm with growth increments ( Fig. 6L ). Lunule broadly lanceolate, smooth. Ligament largely internal, short, set on oblique resilifer. Hinge: right valve with single cardinal tooth and prominent anterior and posterior lateral teeth; left valve with two cardinal teeth, the anterior larger and sockets for anterior and posterior lateral teeth. Anterior adductor muscle scar, short, broad, ventrally detached from pallial line for ca. 15 % of length, posterior scar ovoid. Pallial blood vessel scar visible. Pallial line discontinuous in small blocks. Shell margin dentate, coarser towards anterior and posterior.

Distribution.

Known from the northern Arabian Sea and southern India. Probably more abundant in the northern Indian Ocean but unrecognised.

Remarks.

Dekker and Goud (1995: 6) excluded D. cypselis from Divaricella more correctly suggesting placement in Pillucina . Whereas previously we included Divaricella cypselis in the synonymy of Pillucina angela because of the similarity of external sculpture ( Glover and Taylor 2001) we now recognise it as a morphologically distinct species. The holotype and other specimens from the Karachi area all have a very short anterior adductor muscle scar ( Figs 3 , 7 I–K ) that is barely detached from the pallial line compared with the longer and ventrally detached scar of Rugalucina angela , R. vietnamica and R. munda . It also has a distinctive sculpture of oblique commarginal lamellae set at an angle to the ventral shell margin that is not seen in other Rugalucina . Similar radial ribs and dentate inner shell margin are present in Pusillolucina species, but they have multi-cuspate posterior lateral teeth and lack the oblique commarginal sculpture.

The obliquely inset internal ligament indicates placement in the Loripes group of genera and while the external sculpture resembles Rugalucina in radial ribbing the obliquely aligned commarginal lamellae are similar to that seen in Lucinella divaricata (Linnaeus, 1758) from the eastern Atlantic but that species lacks any radial ribbing.

Figures

Figure 3. Internal drawings of left valves of Rugalucina and Pusillolucina species. Not to scale.

Figure 6. ' Rugalucina ' cypselis (Melvill, 1918). A-D Holotype Divaricella cypselis Melvill, 1918 (NHMUK 1921. 1. 28. 92), Karachi, L 5.2 mm E-L ' Rugalucina ' cypselis Karachi (NHMUK 20191075) Winckworth collection E right valve, L 3.1 mm, ob - oblique commarginal lamellae F left valve, L 3.0 mm G right valve, L 3.2 mm H dorsal view, L 1.9 mm I interior left valve, L 3.2 mm J interior right valve, L 3.2 mm K interior left valve, L 3.0 mm L protoconch M detail of ventral margin (F) showing obliquely aligned commarginal lamellae. Scale bar: 100 µm (L); 1 mm (M).

Figure 7. ' Rugalucina ' cracentis sp. nov. A-E Lucina concinna Holotype (ZMC I. 100470) Gulf of Suez, exterior and interior of right and left valves and dorsal view, L 9.2 mm F-H ' Rugalucina cracentis ' exterior of right valve and interior of right and left valves Egypt, Dahab, Gulf of Aqaba, Red Sea. H. Blatterer colln, L 6.7 mm I, J R. cracentis exterior and interior of right valve Dahab, Gulf of Aqaba, Red Sea, H. Blatterer colln, L 8.5 mm K, L R cracentis exterior and interior of left valve Egypt, 4 km north of Port Safaga, H. Dekker colln 6930, L 8.3 mm M interior of right valve, L 8.8 mm N, O exterior and interior left valve Egypt, 4 km north of Port Safaga, H. Dekker colln 6930, L 7.1 mm P, Q interiors of right and left valves, H. Dekker colln 6930, L 8.7 mm R, S hinge details of P and Q.